I have never felt more 'in' a movie than this one. I actually 'missed' it for days afterwards. So I went back with my girlfriend and watched it on IMAX 3D instead. It then went from Excellent to 'In A World of Its Own'.
The character development was second to none. Only one comment was out of place (Colonel Quaritch's words about 'fighting terrorism with terrorism'). It made little sense to accuse the Na'vi of terrorizing the Humans when the latter were destroying the Na'vi habitat for access to the unobtainium underneath it. Beyond that, it was a perfect movie.
I should close by adding that the real difference between 3d and IMAX was the sound system (i.e. I never knew sound clarity could make so much difference ina movie either)