Even if it were all snow (I doubt it: Skyrim isn't Hoth), it would still be interesting to see how the Nords adapted to the environs and such. Still, I expect variety. You might have been pardoned for thinking Vvardenfell was nothing but ash before Morrowind came out, but we were pleasantly surprised by the variety. Oblivion's Cyrodiil was disappointing, but I'm sure they've learned their lesson. I'm not worried.
I didn't think Cyrodiil was disappointing in the least. There were mountains, plains, forests, and snowy areas. Perfectly diverse. That's what I loved about it. My fear is that Skyrim will be mostly, is not all, snowy mountains. We won't even have the option to go to warm jungles and forests.
Well I think they need to go even more diverse this time. In Oblivion, the only real mountains were around the edges, and there weren't enough areas that weren't forest.
At best, there'll be desert towards Hammerfell, snowy mountainous regions in the middle, and forests and rivers and greenery towards Cyrodiil, Morrowind and High Rock.