To put things in perspective, this is a beautiful effects-driven film. The CGI is seamless and IMAX 3D... wow, just an incredible movie experience. This is the first film I've seen in a very, very long time that I wouldn't have preferred to watch at home.
However, I can't give it five stars for two reasons.
First, the plot is essentially the same as Dances With Wolves. There's nothing wrong with that, Dances With Wolves was also a great film, but Avatar doesn't add anything to the story.
Second, Hollywood has a tendency to be a little preachy at times, and Avatar is no exception. A beautiful film like this has no need to bludgeon you with a "message", and it was distracting.