I don't know, obviously I'm just speaking from a personal point here, but I thought that Avatar actually benefited from the 3D. When I saw the CG on commercials and stuff, it looked pretty good, but I thought it actually looked better when it was up on the big screen in 3D. I think Up did a really good job with the 3D. It didn't necessarily make the movie leaps and bounds better, but it was a bit more immersive seeing everything in 3D, and it made it easier to lose yourself in the movie.
I am going to have to disagree and say that the 3D did in fact make the movie leaps and bounds better.