I think it’s a great movie BUT... they didn't push the envelope. It just didn't really peak. There are a couple of reasons why this is not the best of the best.
The story is solid. Pretty much flawless. But a few things are missing.
1. WHO is Jake?
2. What makes him special?
3. WHY does he change sides?
4. What was his life before?
5. WHAT drives him to his highest peak of emotional level at the end?
James Cameron uses most of the time to show the audience a whole new world (and actually he does that pretty good) rather than to show the audience new people. The Story is foremost about Pandora and not about the people we follow on screen. That’s the difference to Last Samurai. In Last Samurai Tom cruises character actually develops. In the beginning we get to know his character and during the movie his character changes based on the experience he makes with the samurai. In this case it's the experience of love between people and a purpose in life. But JAKE never really makes any experience. He gets to know new people and simply decides to change his mind. After all, the emotions in Avatar are not on the same level as they are in Last Samurai or Dances with wolves. Because we simply don't know enough about the characters. So what does that leave us?
Nothing but a great adventure and a great experience. So, Definitely it can’t be called as The Masterpiece (like the one we are still saying for Titanic), But it’s definitely a good experience after all.
About the CGI and Effects; Ofcourse it is certainly a Groundbreaking work, but I am not that kind of person who considers a movie to be a masterpiece because of its Groundbreaking CGI. Don't get me wrong. But that’s the just you have to be.
Overall I will stick to 3.5 / 5