I hate:
Knee wigglers/chair kickers: They shake any entire row with their lack of patience/control.
Forced laughter, Talkers, Loud complainers
Phone people: Be it texters with their bright screen flashing everywhere, or people actually on their phone having a conversation (like when I saw Order of the Phoenix).
Bad parents: Parents with no control over their kids, and when you complain, they look at you as if YOU'RE wrong.
At the risk of sounding racist, I've generally noticed (in Machester, NW of England) that Indian/Asians are the worst behaved with short attention spans, and are addicted to their phones.
My stupidest experience was Matrix Revolutions: I booked a particular pair of seats for me and my girlfriend. A guy was sat in my seat with a spare either side. He wouldn't move either way. I asked for him to show his ticket (which I knew he wouldn't have) and he wouldn't budge. What a tit. Things were starting to get heated until my girlfriend intervined.
The guy ended up being escorted out of the cinema by policemen and looked even more foolish. Over one seat.
There are unwritten rules and guidelines concerning the cinema and general good manners which are being forgotten all the time, and despite what I said earlier, they aren't defined by age, race or class. It is totally random.