Well, she is squishier and less mobile than other AP mids, and all of her abilities are skill shots (kinda) so if you miss a binding or didn't hit the guy that was low with your ult it's always your fault the team fights go south. I think that they also resent that you are usually the one who survives encounters due to your range, and you are in a weird spot between dishing out damage and being support. There are AP champions who do each of those things better: mobility, chasing, initiation, support, burst damage, so she is kind of middle ground I think. Maybe it's the lack of competitive play she sees too.
I always liked her because she can push, defend and you can take her CDR to insane levels, though in bad hands this just means she is spammy as hell.