I started reading the books once. What put me off was Bella's inexplicable hatred for her father, especially her childlike insistence not to call him dad (though to be honest, I call my mother by her first name, though it's not out of spite).
I love my parents, and have none of the hate of authority that all peers seem to have. Whenever I'm around somebody complaining about their parents, it makes me uncomfortable and, more than anything else, sad that they're unable to connect with the people it should be easiest to connect to. So that's why I didn't like Twilight.
EDIT: Oh yeah, and in terms of the film, what was up with the climatic battle? Big, epic vampire battle, then Ashley Greene shows up and rips off the bad guy's head. Took her about half a second. Did Edward not think of this?