1. I hate being on a team with people who specialise in setting up a spawn-killing or other incredibly cheap scenario. This happened particularly in BFBC2, where occasionally I would get a team who, once winning, would obliterate their way to the enemy base, surround it with all the collective tanks and helicopters, and not let anyone on the opposite team take a step. In BFBC2 it was especially cheap because players searching for a game would automatically be placed in it, the entire team having left.
2. More petty on my end, but I don't like it when I'm in an MMORPG-type situation (I say this because I mean Dragon's Dogma) and other people have ridiculous characters with stupid names and deliberately stupid appearances. Like in said Dragon's Dogma, where I saw a pawn for hire with purple hair and bright yellow beard named Mvfkr. Something unique and different is refreshing, but something deliberately stupid is immersion-breaking.
3. People who will accuse you of being cheap when you are not being cheap, and similarly, people who accuse you of using cheap tactics when you are doing the logical thing to do (e.g. using a health point advantage to force the enemy to come into a closed space rather than engaging them in the open, because you are CQC and that is what you are meant to do).