What 3D is great for is giving you a sense of scale. Say character - building scale. There is a scene in Avatar when they are about to land first time on that planet and they are flying over some open space mine. There is a huge machinery doing the work and I was just blown away how you get a sense that the mine is huge and that machinery is enormous. Sometimes you get a feeling that you are kind of being there on that planet yourself.
I've watched the movie in 2D again, but that didn't give me the same sense of scale. It was like, well watching a movie with cool CGI.