There's a simple answer to that one!

The way that the characters are defined, not only in contextual, graphical detail but the sheer indulgence in to the mammals' backgrounds amidst one another in confusion and befuddlement. Not to mention the insight in to their inner emotions that the audience is encouraged to become a part of - sharing in the untold sorrow of the animals whose winter wonderland of joy and memories is fading away, due to unforeseen geological and atmospheric conditions that rogue across the surface of both hemispheres.

I've just watched them the once, though.