For anyone out there who hasn't seen the movie or read the books, be forewarned, I will be talking about characters and scenes that I hated!!
Two colleagues (one a Twilight fan, the other just likes vampires) and I made a day out of going to see the movie on Saturday. We bought our VIP tickets, went to lunch, had a drink, did a wee bit of shopping, went to the movie.
After the movie, the Twilight Fan (TF) and I were so disturbed we had to have another drink.

First and most importantly: WTF was wrong with the actor they picked for Jasper??? His eyes, his hair, his every odd movement and action...oh Lord. TF and I immediately turned to one another and started laughing.
Every time he came on screen, we could barely control ourselves - seriously, what was wrong with?? Someone needs to tell hair and makeup that the Cullens are the good vampires, including Jasper. I only hope he loses some crazy in the next three movies!
Secondly, we hated the choice for Carlisle. I had heard that Peter Facinelli was portraying him, and I actually thought it would be okay. I like PF, I've seen him in a few movies, and I thought it would be fine. It was not. He had horribly dyed blond hair and looked nothing like my mental pic of Carlisle. TF agreed with me, but my other co-worker, the non-Twilight fan (NTF) actually really liked him.
I was okay with most of the casting selections. I really liked Bella and Edward (although TF and NTF both hated the choice for Edward), which really is the most important thing. I also liked Charlie and Jacob. I did not like Billy - he had a funny, crazy eye thing going there for a scene or two, also.
TF and I are both people who MUST read the book prior to seeing the movie, and then never like the movie very much as a result, because Hollywood never stays true enough to the book. We always go on opening night to the newest Harry Potter movies, and then rip them apart afterwards, but this was worse than that.
Back to Twilight - the other things that I hated had to do with the content. First, it drives me CRAZY when they do things out of order for no particular reason. They did this a lot with the sequence of Bella & Edward's relationship. I hated that they cut parts out of the "meadow" scene from the book (and that it wasn't really in a meadow) and I think if they had left it verbatim, viewers who hadn't read the book would have had a better idea of why Edward and Bella were so drawn to one another. This is my favorite part of the book, and they totally ruined it in the movie.
Also, I hated the scene when Bella was supposed to be leaving Charlie. Where was the passion? The tears? The slamming of doors? Instead, she calmly slams one door, moves back and forth between the bedroom and the bathroom, and doesn't really give Charlie a reason for leaving. She doesn't even cry!! How are we supposed to know how broken up she is about leaving if she can't even shed a tear? Grrr....
Okay, I need to stop before I pick apart the entire movie! The sad thing is that I know I'll go see the next 3 when they come out! Hopefully I like them better :)