I watched the movie again last night and there was one changed sequence in particular where I disliked the changes. Like a few people upthread, I didn't like how they changed the scenes immediately after the baseball game (corresponding to "The Hunt" and "Goodbyes"). In the book, it's Bella who comes up with the basic plan for her to leave Forks and go to Phoenix. It's Bella who intuits that James follow Edward wherever he goes, and that she and Edward have to separate. For me, that segment is a big one in character development, and a big part of why I find her such a good heroine. Even when she by rights ought to be scared completely out of her wits, her mind is still working--in fact, she keeps her head a lot better than Edward does during that episode.
And in the movie, almost all of that is changed to make Bella totally passive. What annoys me the most is that this isn't a case where stuff is left out of the movie for the somewhat defensible reason of keeping the running time under 2 hours. This is a case where the scenes from the book remain in the film, but they are re-written in a way that isn't true to the characters.