What cannot be undone? On which choices pay the attention?

What cannot be undone in the game or costs too much to be undone? On which choices pay the attention to not regret in future?

Race, profession and personal story choices can't be changed unless you delete the character and start all over.

Salvaging or mystic forging items can't be reversed.

Selling items to a vendor can only be reversed up until you change maps.

Selling items on the trading Post can't be reversed.

Character names can be changed but cost 800 gems or so via name change contract.

I'm not sure if any of this answers your question, but I hope it helps at least.

There's quite a few decisions which can't be changed. In addition to the ones Kako listed any item you soulbind/account bind can't be unbound, any story reward you pick can't be swapped for another option (but you can get different ones on another character), in most cases gem store purchases can't be changed, gems you buy can't be refunded....there's a lot.

If you can be more specific about the kind of decisions you're worried about we could let you know which are permanent choices and which can be changed.

while adjustable, some other irreversible choices are:

• Joining an order in the Personal Story (fixed by simply having three characters)
• Choosing a strap in Heart of Thorns (again, fixed by replaying with other characters)
• Likewise with choosing a banner / group to support, in Path of Fire

these are all choices, that are remedied by replaying with other characters.More permanent losses would include;

• Consuming a skin, you lose the ability to sell it, and you lose the free transmutation charge. So only consume skins if you're shy for inventory space.
• As said before, salvaging, selling, and acountbinding cannot be reversed.
• Deleting a character, now I call this irreversible until a point. if it was a mistake, and you contact anet, they can fix it, but only if it's been recent.

Just whenever in doubt, always check wiki or read carefully any pop-up screen you get.

I'm going to restate the lists made by @Kako.1930 & @"Danikat.8537" to divide them into these categories:

• Things that permanently affect your account
• Things that permanently affect a character
• Things that permanently affect items.

And within each category, some things can be adjusted or replaced if you are willing to pay for it.

Account Choices

• Linking your GW2 account to a GW1 account
• Any "upgrade" paid for with gems or RL cash, including getting an expansion, adding characters, outfits, adding bank tabs, etc.

Character Choices

• At character creation: race, profession, and background.
• Cost to change after character creation: gender, looks
• Deleting a character.
• Choices made during the stories, including which Order to join, which race to defend, who to endorse during the PoF story. None of these have a lot of impact, usually just a little bit of dialogue here or there.
• Certain rewards/collections have to be completed on the same character who started them (but that matters little after completion).

Item ChoicesI'm going to say that none of these are important in the long run, because they can be replaced later, if you're willing to spend the coin. A specific item might become bound to a character, but 99.9% of the time, you can get something that does the same thing.

A big trap countless of people did step in over the years (me included) is the "unique" in the description of ascended trinkets. Especially when you are going for your first set of ascended gear, this can be a huge throwback and quite frustrating.

What means unique?It does not mean extremely rare or expensive ^^. It means you can only equip one item of the same name at once. If you check the trinkets at an NPC vendor, you may notice that there are always two items with the same stats but different names. For the Living Story Trinkets, we have got several sources for every piece as well. This is to avoid the issue, but it is never explained in the game. So many people just pick the stats they want, buy two identical rings or earrings and learn it the hard way. Again this is quite frustrating when you are doing in the first time and are extremely short in currencies.

Another thing which was also mentioned above, the tribe you protect and the order you join during your personal story. We are talking about the level 1-80 storyline of the basic game. During the quests you will have a choice of three different orders and tribes to aid in your conquest against the mighty dragon. As @Illconceived Was Na.9781 explained, it is not much of a big deal. But once you begin collecting gathering nodes for your home instance, this could be a little interesting. The reason is, after you have completed certain steps in your story, a representative of the tribe/order will join you in your home instance. That means, you are going to see this NPC every day, for years ^^. Luckily those decisions are character-bound, not account-bound.