Apart from the covers there is no difference between the books.
220.127.116.11 19:55, April 4, 2010 (UTC) says:
On a slightly different note, I personally think that at least volumes 6 and 7 have nothing to do with children's literature at all. Which is not to say that they're not very good "young readers'" books, but I think the books age with the reader - or that at least the reader should age as they progress through the books. Not only for the decidedly dark tone in the later ones, but also beacause some parts of the books' final message are, I guess, only understood in full context intended by adults (or at least teenagers) - that is, if I did get the full message intended :-). But I daresay I got the crucial point, and it was really nicely made.
I agree with the above explanation, but would like to say that JKR knew that her readers were growing up and that adults had started to get hooked on her books as well. Which is probably why there are adult Harry Potter novels; so that the adults who enjoyed HP wouldn't have to lug around hardcover, cartoon-y looking books. :)
I was 10 when I finished the series and I got everything except How exacly Voldemort died, If nagini was a horcrux or not, and if scrigemour was a vampire or not. I think they are all children's books. I didn't even realize there were adult HP books. Why would they just change the cover, I mean isn't that a bit stupid and a waste?