• Valve (the operators of Steam) usually take an up to 20-30% cut of all sales.
  I don't know how NCsoft (publisher) handles this, but I imagine that it means less money going to ArenaNet (developer) and back into the development of the game.
  It might not affect you directly in the now, but this is the biggest issue for me.
  
• With a Steam GW2 account, you are bound to Steam for purchasing.
  There are no other options like https://www.guildwars2.com/retailers. You cannot use codes or gem cards from anywhere else. (Twitch Drops still work via linking a Twitch account via https://account.arena.net/applications)
  
• Fewer editions/bundle. The cheapest way to get all content is not available on Steam.
  On Steam, you cannot buy the Deluxe or Ultimate Editions of expansions (see https://buy.guildwars2.com for details), nor the Elder Dragon Saga Collection - Standard, Deluxe or Ulimate.
  
  • Deluxe Editions usually contain an additional character slot and some other goodies for $/€ 25 more than the Standard Edition.
    (The Deluxe Upgrades can be bought on the gem store, regardless of account type, typically for 2400💎. They go on sale with their associated expansions.)
    
  • Ultimate Editions contain everything in the Deluxe Edition plus 4000💎 (gems) and cost $/€ 50 more than the Standard Editions, which is the price of 4000💎 on their own.
    Thus, if you were going to buy gems anyway, this is a good way to do it.)
  The Black Lion Patron Pack is only available from the ArenaNet website.
  It contains 2400💎 for 30 $/€ (the usual price) and in addition 10 Black Lion Chest (lootbox) keys, 5 Heroic Boosters (great to speed up PvP/WvW reward track progress), 10 Transmutation Charges, a Guaranteed (random) Dye Unlock, and the exclusive title: "Black Lion Patron". Pack on Wiki[wiki.guildwars2.com]
  (nothing you need, but if you were about to buy gems anyway, it's nice to get stuff on top)
  
  See the later sections of this guide for more details on Deluxe and Ultimate Editions, and the Elder Dragon Saga Collections.
  
  All DLCs you can buy on Steam are these:
  • Heart of Thorns & Path of Fire (expansions 1 & 2) for $/€ 30
    
  • End of Dragons (expansion 3) for $/€ 30
    
  • Secrets of the Oscure (expansion 4) for $/€ 25
    
  • Janthir Wilds (expansion 5) for $/€ 25
  … which are also available from the ArenaNet website:
  
  
• No account linking of an original Guild Wars account for cosmetic and prestige rewards
  You can't link a Guild Wars 1 account to a Steam GW2 account for Hall of Monuments rewards (Hall of Monuments Rewards Calculator (incl. Demo mode)[hom.guildwars2.com]): Earn special Guild Wars 2 rewards based on your achievements in the original Guild Wars and Guild Wars: Eye of the North™: Unique armor and weapon skins, ranger pets as well as titles for feats achieved in the original Guild Wars. If you ever see someone with the title God Walking Among Mere Mortals in GW2, know that that player has mastered the original Guild Wars.
  (I wouldn't be surprised if Anet implemented a similar rewards system for GW2 → GW3.)
  
• If there is a technical issue, it's not as clear-cut as with a standalone client who is responsible
  If you use a Steam account for GW2, for issues pertaining to account management, access and game, gem, or other purchases made through the Steam store or Steam wallet, you need to contact Steam support.
  For assistance with a Guild Wars 2 in-game issues, or items purchased from the in-game store using gems, you still need to submit a ticket to the Guild Wars 2 support team.
  On an ArenaNet account, you always contact Anet support.
  
• You rely on an additional server infrastructure
  Instead of just on ArenaNet's (and Amazon Web Services (AWS), which host the game servers), you are also dependent on Valve/Steam's servers. Those have been very reliable for as long as I can remember, but more points of failure is never a plus.
  
• You can't use Anet's 2-factor-authentication
  (since you rely on your Steam account's security instead), so never a Mini Mystical Dragon[wiki.guildwars2.com] for you (just adding it to the list to be comprehensive).
  
• When using the launcher standalone, you can keep the install size of GW2 down
  … by not downloading the zones you don't need yet; only the five starter zones (and probably some others) will be downloaded if you set Content Streaming setting to On Demand in the game's Options [F11] (the option is only observed by the game client; the launcher always updates regardless. Conversely, when installing through Steam and using the Steam GW2 account, you have to download the entirety of the last "snapshot" before you can launch the game from the Steam Library.
  (This is a very niche case likely irrelevant for most people, but it's nice to have the option.)

Furthermore, I don't think that the barebones implementation of GW2 on Steam warrants the 30% cut – even though I'm convinced this is the game's publisher's fault, not Valve's.

• no Steam achievements
  
• no trading cards
  
• no Points Shop items
  
• no profile backgrounds
  
• no avatar frames
  
• no chat emoji
  
• no Steam Workshop… that could help with distributing and updating popular addons
  (like BlishHUD, ArcDPS, RadialMount etc.)
  
• no rich-presence… to show the map friends are playing on in the Steam friends list (like the in-game contacts and guild panels do)

Let me stress that I really like Valve and Steam. I believe that PC gaming wouldn't be as good as it is today without them, and that that Steam is great for indie developers.
I just don't think Steam is the best place to play and buy GW2 on.

Are there any advantages to using a Steam account for GW2?

I just want to have all of my games on Steam.
That's an understandable sentiment and with the method described below you can still launch the game through Steam, use the overlay, screenshot manager etc. with it. All you need to do is create an account on the website and save the credentials somewhere. If you use the internet at any capacity outside of gaming on Steam, you already need to save login credentials in some way, so this should realistically not be an issue. You can set the launcher to remember your credentials and activate autologin, so you'll almost never have to use them either. Making a standalone account gives you the flexibility to use Steam when you want, or use the launcher directly if you don't. With a Steam GW2 account, there is no such freedom of choice.

One big thing that could affect you in certain countries is regional pricing[steamdb.info]. Steam can help with currency conversion.
Whether being able to use the Steam Wallet is a pro depends on your preferences.

The only situation in which I believe that Valve really earned the 20-30% cut is if a player has only learned about Guild Wars 2's existence from the Steam Shop.
In that case, I see how it's probably more fair to play on a Steam and buy DLCs there.

In any case, it is not necessary to re-download the game! All you need to do is make an account[www.guildwars2.com] using an email address and a password of your choosing. There are two ways (that can both be used in parallel) to launch GW2 and make it use ArenaNet's login servers, assuming you already downloaded the game through Steam:

1. Steam launch option "-provider Portal":
   
   VERY IMPORTANT!!! Read carefully and understand fully!
   This will NOT link a standalone account with a Steam account. Purchases made via Steam will still be credited to the Steam account and cannot be used with a non-Steam account.
   You need to buy expansions on buy.guildwars2.com or any retailer listed on the official website: Official Retailers[www.guildwars2.com] if you want to activate them on a standalone ArenaNet account.

   ---

   
   This option will run the game through Steam, you can use the Steam Overlay (Shift+Tab), your playtime will be logged on your profile, you'll appear in player numbers and SteamCharts (which mean nothing for this game), you can use the screenshot manager and so on. Decide for yourself if it is morally right to circumvent buying the game on Steam but using its features.
   
   Instructions:
   In your Steam library, find GW2 and right-click it. Click Properties…. In the window that opens, put the following text into the empy text box under "LAUNCH OPTIONS":
   
   -provider Portal -autologin
   Capitalization is important here: If you write, e.g., lower-case portal, it won't work and when you try to log in via the launcher, you'll get the message: "The code you entered is invalid. Please check your information and try again.".
   
   (Only "-provider Portal" is needed to make the Steam install use ArenaNet login servers, "-autologin" is just for convenience. There are more command line arguments[wiki.guildwars2.com] for GW2 one can use.)
   Separate guide about this method by someone else: https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=2853177345
   
2. Standalone:
   In your Steam library, find Guild Wars 2 and right-click it. Select Manage > Browse local files…. Make a shortcut to Gw2-64.exe.
   Using this will launch the game without Steam. Steam doesn't even need to be running for this.

Either launching the Gw2-x64.exe directly, or launching it in Steam with the -provider Portal launch option will make the launcher look like on the right side of the image below, asking for email address and password and giving the option to create an account, which will open https://www.guildwars2.com/createAnAccount in your browser.

Guild Wars 2 came to Steam for its 10th anniversary. Existing accounts made on ArenaNet's website cannot be converted into, linked or merged with a Steam account, while GW2 accounts made on Steam can only purchase through Steam.
(By the way, there was never a good reason for players who made an account before the game came to Steam, to make a new GW2 account there, which is why the player number for the game on Steam vastly underrepresent the actual number of players.)

While you can play the same content regardless of account type and play to together with anyone in your server region, meaning once in the game you won't notice much difference between account types, there are certain money-related differences, which I am going to go into in this guide.

Also see ArenaNet's Steam FAQ[help.guildwars2.com].

At the time of writing (September 2024), there are five expansions and the Living World.
These are the story/content releases in chronological order:

⛲ full list of releases with dates and features[wiki.guildwars2.com]

Everything in the GW2 base game and LWS1 … can be PLAYED FOR FREE with some restrictions: (free accounts)[wiki.guildwars2.com] like fewer character slots, reduced inventory space, some chat and trading post restrictions, and no exchanging of ingame coin/gold for gems. Also, free accounts only get a Trial Mount that lasts for 10 hours (that's probably how they get most people to buy an expansion; the tried drug dealer method)
The free game includes all playable races, 8 of the 9 professions, the eight original dungeons, Fractals of the Mists (dungeons with scaling difficulty), PvP and WvW (Competitive Play[www.guildwars2.com]).

One overarching storyline from start to finish, including the Living World.
The expansions HoT, PoF, EoD are the biggest and most feature-rich ones, adding vast new zones, a new profession (base class), elite specializations (sub classes) for each profession, gliding, mounts, boating and fishing, strike missions and raids … to name just the big stuff.
The Elder Dragon Saga is available for purchase in various collections, only one of which is available on Steam.

While LW seasons aren't as packed with new gameplay features, they add new zones and the story content is essential for the broader narrative, not just less important side story. The stories in the expansions may work on their own, but for the full picture, you'll want to play the Living World seasons too – in chronological order.

After the conclusion of the Elder Dragon, rather than launching an expansion every two to four years with a season of Living World in between, ArenaNet began releasing smaller expansions (e.g., SotO and JW) more frequently at a slightly reduced price (usually $/€ 25 for the expansion itself (Standard Edition) and adding additional content for those expansions through quarterly updates, meaning that the next big release is only ever a few months away.
Secrets of the Obscure and Janthir Wilds were and are released in this fashion.
Anet Update: GW2 2023: Defining the Problem →The Path Forward →What it means for you[www.guildwars2.com]

For the first three expansions, HoT, PoF, and EoD, there is the Elder Dragon Saga Collection in a few different editions:

• Elder Dragon Saga Collection Standard which includes the first 3 expansions
  
• Elder Dragon Saga Collection Deluxe: Standard + some goodies
  
• Elder Dragon Saga Collection Ultimate: Deluxe + 4000 💎
  
• Elder Dragon Saga Collection Complete includes all content releases from the first 10 years, including the Living World Seasons 2–5.

⚠ only the Complete Edition is available for purchase on Steam: GW2:EDS–CC

• Deluxe Editions cost $/€25 more than the Standard Edition and add some goodies like an additional Character Slot (worth 800💎) and unique cosmetics. Deluxe Upgrades are also purchasable on the gem store if one bought a Standard Edition.
  
• Ultimate Editions, get you everything in the Deluxe Edition plus 4000💎
  
  (the equivalent of $/€50) to spend in 🦁 Black Lion Trading Company (the gem store), at a cost of $/€ 50 more than what the Standard Edition costs.
  So, in a way, when buying an Ultimate Edition, you get the Deluxe goodies "for free" or as a bonus.

If one didn't buy the Deluxe or Ultimate Edition of an expansion, the "Deluxe Upgrade" containing all Deluxe goodies can later be bought on the gem store for 2400💎. These are usually also discounted by as much as their associated expansions (e.g., when the HoT+PoF bundle is -75% off, both Deluxe Upgrade bundles will also be available for 600💎).
This option is available for Steam accounts aswell.

WARNING: If you want the Living World for the story, and thus don't want to skip anything, do not buy individual episodes for 200💎 via the story journal of your hero panel!
Instead, open the Gem St[o]re and click on Account Upgrades → Living World, where you will find the "Living World Season Complete Packs" in which each episode costs 160💎, (season 3, 4, and IBS only if you own Heart of Thorns and Path of Fire).

The Living World seasons cost the following when purchased separately via the gem store:

Since the Elder Dragon Saga Standard Collection contains HoT+PoF and EoD,
and the Elder Dragon Saga Complete Collection contains all Living World Complete Packs in addtion, you save $/€ 2, by buying the Complete Collection.

A frugal path for people who want all content releases but at the lowest possible real-world money expenditure (focusing only on Elder Dragon Saga for now, since the newer, smaller expansions, SotO and JW haven't been discounted yet) could look like this:

1. Buying the Elder Dragon Saga Standard Collection for $50 (or $25 on sale) which contains Heart of Thorns, Path of Fire and End of Dragons
   
2. Grinding for the gold to convert to gems to buy the Living World Season Complete Packs for 1280💎(LWS2), 960💎(LWS3), 960💎(LWS4), and 960💎(IBS) (or less on sale).
   At the time of writing, 1280💎 and 960💎 cost 550 gold and 412 gold, respectively.
   (See Exchange rates[gw2efficiency.com] or the second tab of the 🦁 Gem Store in game for current rates.)

If you planned on only buying the expansions that you have to buy with real money, and grind the ingame coin to convert into gems to buy the Living World seasons

• Living World Season 2 Complete Pack[wiki.guildwars2.com]
  
• Living World Season 3 Complete Pack[wiki.guildwars2.com]
  
• Living World Season 4 Complete Pack[wiki.guildwars2.com]
  
• The Icebrood Saga Complete Pack[wiki.guildwars2.com]

as described above, … buying the two DLCs Heart of Thorns™ and Path of Fire™ ($/€ 30) and End of Dragons™ ($/€ 30) will cost you $/€ 10 more on the Steam Store, than buying the Elder Dragon Saga Standard Collection (non-complete, only HoT+PoF+EoD) off buy.guildwars2.com, which you could only get there, directly from ArenaNet.

Secrets of the Obscure and Janthir Wilds also can be bought in Standard, Deluxe, or Ultimate Editions. The deluxe goodies are the following:
SotO: a Character Slot Expansion, an Identity Repair Kit, the "Magic Juggle" Emote Tome, and the Astral Ward Feathered Raptor Skin.
JW: a Character Slot Expansion, an Identity Repair Kit, the Wayfinder's Versatile Table homestead decoration, a set of Eldritch Horror infinite gathering tools, and 3 Black Lion Glyph Selection Containers (🪓 [fast] Farming Community's info on Glyphs[fast.farming-community.eu]).

*especially the older (and, imo, at the same time best) ones.
Note that Ultimate Editions (those that contain 50 bucks worth of gems) never go on sale and even get taken off ArenaNet's store page during sales. The Elder Dragon Complete Collection has been discounted -50% numerous times in the past.
⛲ Sales history[isthereanydeal.com] and Sales History[gg.deals], personal, manually curated list of giveaways and sales by community member Dak393[wiki.guildwars2.com]

Taken from https://buy.guildwars2.com/en-us/collections, for easy referencing:

(Expansions 1, 2, and 3)

With the Standard Edition, you'll get the first three Guild Wars 2 expansions, along with 2 Shared Inventory Slots and 2 Max Level Boosts.

You'll get the first three Guild Wars 2 expansions and everything included in the Standard Edition, plus a Character Slot Expansion (worth 800 💎), an Identity Repair Kit, the Shing Jea Dragon Boat Skiff Skin, and a Canthan Raptor Skin.

You'll get the first three Guild Wars 2 expansions, everything in the Deluxe Edition, plus 4,000 gems to spend at the Black Lion Trading Company in the game.
(Ultimate editions cost the equivalent price of the 4000💎 ($/€ 50) more than the Standard edition, but also include the deluxe goodies.)

This is the only one of these editions available on Steam, and once you bought any part of it, e.g., the "expansion starter pack HoT+PoF", you can no longer get the bundle.
With the Guild Wars 2 Elder Dragon Saga—Complete Collection, you get an entire decade's worth of content! Included are Guild Wars 2 Living World Seasons 2–5 and the first three expansions: Guild Wars 2: Heart of Thorns, Guild Wars 2: Path of Fire, and Guild Wars 2: End of Dragons. You'll also receive 2 Shared Inventory Slots and 2 Max Level Boosts to help your characters get started in level-80 content.

• Living World Seasons 2-5 (approximately 40% of retail value) is non-refundable.
  
• Living World Season 1 is free to all users.
  
• Living World episodes become playable at level 80.
  
• Each of the 2 Shared Inventory Slots and 2 Max-Level Boosts are included as part of the Guild Wars 2: Heart of Thorns & Path of Fire expansion set or the Guild Wars 2: End of Dragons expansion.