I've read all 5 books and I enjoyed it, but some of the things they changed/didn't put in, irked me.

Like not explaining the map (why Lupin knew what it was and what it did and Snape did not. Don't you think Harry would of asked?) or why the patronus that Harry conjures is a stag.

I think if they had kept true to the book and had Crookshanks help the kids with the Whomping Willow and not have them ride it like some freaky amusement ride, then they would of had time to add the other things I felt were important.

I guess the director just assumed everyone who sees the movie reads the books but that is not the case.