A lot of Twilight haters are people like me who enjoyed the series when they were fourteen and then grew out of it. (Also it's just one of those things that are fun to hate. Like new Final Fantasy releases?)
Of course, you may then grow back into it later...
And a lot of people who like "big, bad, and edgy" things apparently don't like "cute" things. I, however, happen to be a fan of both, which makes me an oddball.