The behavior I most dislike is the passive aggressive negativity of someone who has for some reason not actually declined to play, but acts miserable and tries to make everyone else miserable the whole time. This might be a new player who constantly complains or ridicules the game like, "oh my god, I can't believe I'm doing something this nerdy" or a player who prefers a different game constantly saying, "this would be so much better in DnD 3.5."

Another behavior I can't stand is the type of player who is there just to lord how much smarter he is over the other players. This type can be especially bad if it's the GM.