The movies - so far - have been horribly inaccurate.
Book One:
There was a huge scene in which Harry and Hermione snuck Norbert up to the top of a tower so that Charlie (Ron's older brother) and some of his friends could take him away to live with the other dragons. BIG dropped part.
They skipped the poison/potion room before the final challenge.
Book Two:
Nearly Headless Nick's 'Deathday' party, a funny scene, was eliminated. So were numerous other, small, things.
Book Three:
The worst. They showed one match - done inaccurately - of the Quidditch season. This is the book in which Gryffindor won the Quidditch Cup, the biggest quidditch happening in all the books! Many interesting and funny matches were skipped.
Additionally, when Harry's Nimbus was smashed, he got a Firebolt for Christmas, rather than the package at the end of the movie.
Finally, they skipped the train scene going back home, where Harry gets a letter from Sirius and Ron gets Pigwidgeon.