I saw it in the store last week. I couldn't find any huge differences, but the introduction says:
–Vocabulary has been enlarged for all chapters,
–Conjugations are written out rather than having the verb and a number in parenthises like they were in the original edition,
–I think I remember reading that the example sentences were more numerous.
There may be more features. I wasn't terribly impressed, but anyway I am happy with Wheelock the way it is. So I'm sticking to the original 6th edition.
Oh– it seemed like it was printed on better paper, too, although without the original to compare it to I can't be sure.