My oldest son told me this was his least favorite of the Harry Potter flicks.  I'm thinking it's because there was more story-telling than action.

If memory serves correctly this movie is pretty close to the book so I may not have a lot to say here.  With a few notable exceptions:
1. Cho doesn't give up Dumbledore's Army.  With the marked absence of the house elfs from the movies you'd never know it was Dobby that gave him up.  (Read the book if you want more details.)
2. After Trelawny is sacked the school hires a centaur to replace her.  This is the reason (partially) that the centaurs are so pissed.  They consider the hiring of Firenze bad because, well, they just don't like people.
3.  There are no brains in a giant tank in the department of mysteries.

Another thing is Quidditch.  As important as it is in the Potter Universe it seems to be on the backburner in the movies.

Fred and George's exit is much more enjoyable in the movie.

I'm pretty sure we've all had a teacher like Dolores Umbridge:  a know-it-all who doesn't listen to reason, and is just annoying a piss.  I'm pretty sure I was that teacher for a couple of years.