I got the UK books with the "adult" cover art so they look all fancy. Although I do kind of miss the interior illustrations.
One thing from the movies I love is in Order of the Phoenix when Luna says "I hope there's pudding." From what I understand, in the UK "pudding" can be used way more generally than how Americans use it, the same way we'd say "dessert," so hearing the line "I hope there's pudding" as an American makes it sound that much more random and bizarre. And Order of the Phoenix was my favorite of the books and the movie just didn't measure up for me, so I had to enjoy the small things like that.