I use to hate a couple of things. Here's my list:
1. People talking to your opponent while you are trying to conduct a shooting phase.
2. People talking loudly in the room where you and your opponent are playing.
3. An opponent who forgets constantly what he has on the board due to too many proxies.
4. An opponent who tries to remove your casualties for you. (Yes you killed my last three guys, does not mean you get to move them off the board).
5. Opponents who take friendly smack talk to a level of out right denigration.
6. People chiming in their own opinions on how you and your opponent run your armies (especially when their ideas are either wrong or horrible).
Oh I could go on and on.