A lot of people in PUGs (Pick up groups, for those who don't know the acronym. Basically, random cross server parties) just want to braindead DPS, want to get carried, and don't want to be bothered helping to explain mechanics to newer players.
A lot of cases in certain dungeons if a player screws up a mechanic they'll just drop instead of teaching and giving a second chance.
To be fair to them, when you need to farm a dungeon literally hundreds of times for a drop, you kinda just want to get that shit done quick. And even if you're generally a helpful person, 100 runs later, you just don't want to bother explaining anymore.
As a gw2 player, if you were around before the expansion dropped, you might remember this same attitude as the "Zerk only, Melee/Ele only" type groups filling the dungeon finder.
And yeah, this game is very similar to gw2 in that any party/class composition is viable, and all the roles and mechanics are pretty liquid. Your role isn't so much your class but which mechanics you take over. Like for example, in gloomdross incursion and in Sunderned nexus, both of those final bosses will target the farthest player with certain mechanics (Jumps for gloomdross, and Drill missiles for Nexus) so the player needs to know those mechanics, or stay at close range to the boss to let a certain other player be the designated target. Ideally the target could be any ranged class, but it doesn't matter too much in the end.
Each class has a few support skills they're known for, like Walrock's soulburn buff, Sin's group stealth, FM's Ice Sheathe and such, and you should learn which of these skills people will expect you to use and when and how to use them most effectively.
Then a few other classes have some broader specialties and weaknesses, For example: destroyers have a lot of CC, so they might be expected to use specific CCs on encounters that require specially timed stuns or something, they have one of the lowest, if the THE lowest single target DPS against bosses in the game (For the same stats on other classes) but on the other hand they are very good at mopping up adds and trash mobs. Warlocks on the other hand are kind of the opposite, in that they have few good CCs, and are kinda meh against swarms of enemies (Dark dragon helix build is less meh there though) but they can deal massive damage to single targets with enough health to let them ramp up their rotations.
That said, other than a few very visible skills (Again, soulburn, frost sheathe, and class with a pin or grab) people in pugs who don't play your class will barely have a clue what you can do anyways. Again, very similar to gw2 in this respect. Warrior = Banners, Ele (used to)= ice bows, etc.
Even then, all that isn't so much a "role" per say. In the end, it's a LOT like gw2 actually in that it honestly boils down to: Everyone DPS as hard as you can, don't die, and learn which boss/dungeon mechanics will trigger bad stuff like full party wipes and don't screw those up.
Oh and general advice, if you don't want people to salt at you, just never knock anything into the air in a group setting. You'll almost always *cricket* off a melee player who whiffs a ground-based DPS skill because of it. (A coordinated group can actually use well timed air combos well, especially in 6v6, but eh, that's an argument for another day)