To those with a problem with the 3D/2D disc differences...
My understanding of 3D is this. You must keep as much of the picture IN FOCUS as possible, to create sharp images that the 3D can work with.
Now, with traditional 2D filmmaking, having your backgrounds be soft and out of focus is a style. It has to do with depth-of-field. Animated films have been trying to recreate this, especially over the last 10 yrs. They even hire film DP's to advise on such things (like Roger Deakins did for Wall-E).
What's fairly obvious with this animation, is that to create a better 3D experience, they sharpened all the backgrounds that were originally softened for the 2D release. For 2D, they simply wanted the best image possible... for 3D, they had to create the sharper backgrounds to help the 3D 'pop'.
Short version... the 2D version on the 3D disc is the gimped version. If you're watching in 2D, I'd bet dollars to donuts that the director's intent is for those backgrounds to be soft and out of focus.... not sharp and overly detailed.