The story of Avatar is more entertaining than the 3D presentation thereof.
I dont think anyone, but maybe FLY, was all that impressed by the 3D part. It is the VFX that are amazing in the movie. The 3D adds a tiny amount to the movie, but I think it would be equally visually amazing in 2D (although to be fair the two times I saw it were in 3D so I dont know for sure).
For me the movies enjoyment breaks down like this: