Unfortunately as with most book to movie adaptations the producers and writers either have to completely rewrite the story, or cut out unimportant scenes due to serious time constraints. 600-800 page books will always include more detail, and more character I think, than any movie ever will. It has improved now that the movie industry is starting to accept 3+ hour movies a little more; mind you most people won't sit still for 2.
Cue in the Harry Potter series. I love the books, as strange as that seems to most older folks and young adults, it includes a lot more in-depth material than most anything else these days. I think the greatest thing JK Rowling did with the series is center around the idea of death later on. Most parents sometimes shelter their kids too much about certain ideals, and I think subtle ideas such as the ones in the Harry Potter series provide a better understanding for the younger generations.
The movies are getting better, but I personally don't like them. The main character of the entire series who plays Harry Potter, Daniel Radcliffe, I must say is a terrible actor. Does anyone else notice? It seems like he is trying much too hard to act in so many places, that he just ends up over-acting the scenes and ruins them. The supporting actors are certainly not the greatest around but their acting is more solid than Daniel's. I find it hard to stay focused on a center character in a movie, that I really can tell is faking every emotion. I mean provided as the series goes along he does get better and better but...
The worst is when he has his little Voldemort induced mind attacks or when he gets angry. The kid just sucks in his breath harder and harder and opens his eyes wide...I mean, a squirrel could do that. Am I the only one around that thinks they should have cast a better Potter?