Why do people hate it?
It's very, very popular, and pretty much defines the mainstream game.
Ok, that's not all of it, but it is not as bad as people make it out to be. There are server problems and the campaign is too short. The online is supposedly unbalanced, but I don't understand that. So long as you're somewhat capable at shooters, the online is fun. I've never found myself the victim of 'n00bs' an unreasonable amount, and I'm barely a great player. The maps aren't great (not as good as MW, and nowhere close to WaW), but the bare fact remains that it has the best splitscreen ever.