Those dorks in big team matches (5 or more) that start teamkilling when the chips are down. I suppose they figure that there's no way we can catch up and win, which is a perfectly acceptable (if a bit pessimistic) attitude to have. When they start assassinating teammates as soon as they spawn, though, is when it becomes a problem.
Fighting game players who spam one move endlessly, ie, Ryu's Hadouken, Kilik's multi-poke, Liu Kang's Bicycle Kick, Fox's laser. It's not that this strategy makes them particularly difficult to beat, it's just annoying.
Achievement whores. Generally, I don't have a problem with gamers who like to beef their gamerscore by getting every last achievement. It's when they come into Matchmaking begging everyone to help them boost that I get annoyed. On the plus side, it usually provides me with a convenient whipping boy for the duration of the match.
