Minor differences - no Eowyn Faromir romance for instance. Aragorn does not call in the Rangers - far as we can tell from the movies he's it. Aragorn doesn't get his sword until this movie. No Scourging of the Shire. Some of these changes are huge to Tolkien geeks, but none of them have bothered me too much (well, except that whole falling off the cliff thing from TT - unnecessary). But no big spoilers if you have read the books. However, not everyone has read the books.