The only one that persists beyond any one game that I loathe is Spawn Trapping.
I mean, fucking seriously! I just hate games that have set spawn areas that people abuse to no end. BC2 isn't bad in this regard (Because when I play, I get matched up with people with senses of justice, seeing as they aren't on MW2), and Halo usually has spawn points all over the map to counter this. But there's nothing that makes me angrier in online play than spawning, walking forward and being shot by someone making sure our whole team is unable to act.