Cocky people make me sick to my stomach... I mean, friendly competition is good, I mean, saying stuff like "lol, I just owned you with a grenade" is OK, but "OMG, you all suck at this you complete noobs. I swear to god I am amazing at this, fuck yeah, no scope!" is just stupid. When people get arrogant, it's annoying, even when team-mates do it. There's a fine line between competition and being big-headed.
Also, teabaggers. I don't think they realise they're pretending to rub thier balls in someone's face... Idiots.
I have to say, I don't mind griefers, but I hate the people that respond to them. I have done my fair share of griefing, and while it isn't nice, it's helluva lot of fun. I mean, you tell me putting someone off thier sniping isn't funny just as they get a shot lined up. The way I see it is that it's pranking in games.
Also "Hardcore" gamers. Bunch of fags. Since when weren't games made for fun? All hardcore gamers care about is winning.
Oh, and people who use the word "clan" to describe thier team in games. I don't like people who call it a clan, a "Team" or "Group" or "Guild", sure, but never a clan...