if you go into it thinking it's going to be better than the last one, you may be disappointed. Kind of like Return of the Jedi when it came out. Still an excellent movie, but when compared to Empire, it doesn't look so good.
Essentially, it's not often movies are as good as The Dark Knight, and expecting this next installment to be on that same level might open you up for a letdown, even if it is still a pretty good movie in it's own right.
(I'll be going to see it sometime next week, and my expectations are through the roof, so I suppose this post is as much for me as anyone else)