Here's some of mine:
1. weapons/items/cosmetics with stupid names: yes, yes freedom of speech and all that but if you feel the need to name your weapons after genetalia or other stupid shite then go french kiss an angle grinder, it's just plain annoying.
2: people who taunt after killing: wastes time and makes you look like a douche, just don't do it.
3. ragers: keep your impotent anger to yourself, we don't want to hear your ehining about who's hacking or who's good or bad. shut it. "BUT LE MUTE BUTTON--" shut up, mouth shut! I don't want to have to waste time trawling through the mute list and otherwise having my patience tested by these cretins. shut it!
4. unreasonable traders: I am selling X item for Y amount of metal, shall we say. trade offer. lowball
offer 2: lowball
offer 3-7: lowball:
offer 7-12: unrelated to what i'm selling
THIS ACTUALLY HAPPENED.
I have clearly stated that the price is non-negotiable, pull this nonsense with me and you get disconnected.
5. Idlers: if you have other things to do then that's fine, but if you're going to be gone a while then disconnect. you're a load on the team and server that could be used by someone actually useful. what's worse is when they join, do nothing, then disconnect X amount of time later, this is a real issue on trade servers. one time a while ago i went on a server with 14 people on it, not one of them was active.