The one thing I find to be the most irritating is how mean older/more experienced players can be to newcomers. You can't expect someone who just picked up the game to automatically already know everything about how to successfully play a match of zombies. Sure I won't be too enthusiastic about playing with people who have only gotten to round 5 or so, but I'll be more than willing to help them by giving them any kind of tip or piece of advice so that they can become a better zombie player themselves.
Also, I do find box hogs to be really annoying, especially when I'm down and someone picks up a fire sale (which usually results in the end of the game anyways) and completely disregard the other players to spam the box.