I hate spoilt brats, a classic example of this is that I know a girl (girlfriend of a family member) who has spent her whole life being spoilt and pampered my her parents. A few months ago her parents said they would buy her a new car, then it turned out they had a few money problems and told her they couldn't afford to do it for another few months, her response to this was to pull up at the side of the road, get out of the car, lock the car with her parents still inside and walk off and leave them for about 2 hours, if thats not a spoilt selfish brat I don't know what is!!

I also hate it when people are happy to bitch about something you like but when you say bad thing about what they like they get an attitude problem.
Also hate it when people over-react. A friend of mine is put something on her FB durin an England match, saying "Oh so this is whats it's like to be a single mum" all because her partner had gone to the pub to watch a bloody football game, then started going on about how the enagement was off cos he didn't paint a bedroom when she told (not asked) him to.