And I haven't read the books.

The girl seemed to much of a throw away character. No depth. No personality. No anything. Just some announced relationship to the main characters. In any other series (say "Star Trek" or something) I wouldn't ever expect to see her again in any other episode.

Also, the bit with the car was never explained. Why did it keep showing up to save them?

As for Tom, I find it difficult to believe that he'd make an anagram of his name and use "I am" in it.

And doesn't Harry know more than one spell? Even the comic side-kick attempts more spells.

It's not a bad movie. The snake was boring. The spiders were good. It was based on a kid's book. It just seems to be missing a lot of material.