God, there's a bunch. So I'll just pick the latest one that came out that I saw.
Potter 7 part 2 was way better than the book. It told the story better, I thought. I really noticed the relatively complex relationships between Harry, Dumbledore, Riddle, Snape, Lily, Draco, etc, in the movie, but none of that seemed very important in the book (even though it's kind of, you know, the driving force of the story). Also, Snape's big final twist thing was really not believable to me in the book, it just didn't seem like something he would do, but watching the movie I really felt it and believed it, so I think it was really well done there. I guess this is mostly due to the fact that Snape's actor is really good at his job.
Dumbledore, as anticipated, has a terrible actor in the movie, but somehow in this one it works to his advantage. It really helps to characterize him as this godlike, unfeeling being, who doesn't care about individual lives or emotions, but does everything for `the greater good' (a term used in the book but not in the movie, as far as I can recall). In the book, this concept of the greater good is not expressed very well by the Dumbledore character, because he does care about Harry, he apologizes to him, and he generally acts like a friendly, loving guy, but in the movie none of that can be expressed through the stone cold, blank expression and the flat, emotionless voice of a horrible actor. I really got the feeling that, in his all-powerful omniscience, Dumbledore had truly forgotten long ago what the fight between love and fear, good and evil, was actually about, and this makes him an interestingly flawed character.
The only misgiving I had about that movie was that Ginny got like zero lines. Ginny's awesome and should get to say something ...