Well, the reason why the first movie seems to be so faithful is because the author actually wrote the script for it. (She had some help, but she did most of it)
Now, the second movie will have a different writer, so we'll have to see about that.
Plus, I'm wondering, considering that the movie is rated PG-13 and not R, how the hell are the going to handle a lot of teen deaths? @_@
I mean, the books were pretty..well, graphic about it.