Don't play much multiplayer games other than Ghost Recon Online nowadays, but based on that...
Little things:
-When there are people communicating in a language that's not my own on the team audio channel, who then get annoyed with players for not listening to them. If you're playing with friends, that's cool, use whatever means is easiest to communicate, but don't get shitty with me or other players when we don't know what you're saying (apart from when you can suddenly say "Stupid f***ing noobs").
-Those players who go on rants about "noobs". I was going to say the players who call others noobs when they are low on the match's ranking, but anyone who diverts to that kind of language to blame their own team-mates.
-Sitting way back, pretending to be a sniper, when we need you on the point. Need bodies on the point to capture it, faster with more people, we just killed their entire team, we have 1 person on the point and there are 3 snipers sitting 25 metres back, waiting... Part of the "players who are only interested in their own development/survival vs the teams goals/objectives.
Big thing:
-Hackers. Bloody trolls. Their glee when their team wins, as if they did something of worth... Thankfully doesn't happen often.
EDIT
Hero of Lime said:
People jumping ship in a Halo match because they are on a losing team. It makes a losing, but manageable situation into a massacre once the team gets too small. I'm sure some of them would leave for legit reasons, but you could tell it mostly because they didn't want to stick around to lose.
Also, this.... You get into a match, play the match to the end, battles can swing the other way just like that sometimes, and other times you just gotta stand through the storm. Thankfully GRO has started tracking match completion rates for people, so you can see if someone is flaky...