Up - The animation is fabulous. The Avatar/Driver premise is brilliant. The world of the Na'vi is spectacularly created. The subtle use of 3D allowed you to feel that you were there. The acting was exactly right for the film. (It would have been hammy in almost any other type film.)
Down - you knew the story from the beginning because you've seen it a million times. The characters were predictable and some, particularly the military, were too one-dimensional. It became tiresome to see over and over again about just how lousy almost all humans are while other species are ideal.
The positives here are FAR greater and the whole is a very good movie.