As is his habit, James Cameron has produced a movie that touches the viewer on a number of levels.
The visuals are stunning. The CGI is cutting edge (especially the motion capture of the actors) . The story is enthralling enough that the length is not a detractor.
The 3D was a perfect addition to the movie. In other 3D movies I've seen, the director seems to go out of his/her way to remind you that you are watching 3D. Mr. Cameron used the 3D to put you inside the screen...either with flies buzzing about, or dust falling around you. Rarely do objects fly out at you.