"illegal" hovering
I also hate when I yell "Coming in" and just then two guys taxi out to the runway for takeoff. Last week I just about had it and expressed my opinion. Being short tempered, things like that push me past the point of enjoyable flying to the RC equivilant of road rage. It's just plain rude and a blatent sign of disrespect.
However, if you've got the field to yourself with no-one else on the flightline or in the air then by all means have some fun. That's what it's all about. Just don't hover 10-feet in front of me while I'm trying to pay attention to my model 500-feet away. I can't pay enough attention to your model to feel safe while I'm trying to fly.