I've got a healer and see a LOT of things in normal because I don't have a lot of wait time.
Things that annoy people are
1. Pulling aggro when you don't have the DPS or heals to handle it so you keep dying.
2. Starting boss fight aggro before before everyone is together (if you start a fight before a member gets through the door/gate, they won't be able to join the fight. that means less DPS)
3. Mostly pulling too much aggro and dying all the time as a TANK.
Not much is required of a tank in normal mode. If you plan to tank, poke the boss (distract it) and stay alive. Don't cause the rest of the group to die because you're impatient.
If you think it's a solo act and don't work as a team member, I'll leave the group or just not heal you.
I see people marked as DPS go in with a frost staff (that pulls aggro) and other things that just attract aggro instead of causing damage when they're DPS. (Mostly an issue in Vet)
The other day, there was a guy who came in and just stood there. Did NOTHING. He had up lightning form and just stood there.
We noticed so we stopped our DPS. You're in a group, do group stuff.
And for goodness sake contain your pets, if you have them. If you don't disable them when not in combat, they will start fights with anything (they're good tanks for single target bosses though).
[EDIT]
This is incorrect. Pets do not aggro unless a player has entered combat first. Common misconception but unless a player enters combat pets will not 'start' fights.
Thanks,
@Princess_Ciri
In VET:
1. Assuming everyone knows the mechanics and just jumping in (when they make a big difference)
2. Not having food on you
3. Being a build (DPS, Heals, Tank) and saying in the group finder that you're something else. Vet isn't normal mode, mmmkay?
4. Same things I posted above about normal. Just makes things more difficult.
5. Don't read the effing scroll (learned that the hard way) unless your group has enough DPS.
5. You need good DPS in Vet. There's always a DPS check.
6. Stay out of the RED CIRCLES. They're usually one-shot kills.
For mobs, use encase/talons or some kind of roots so you can kill them without them doing much to you.