I think regardless of what Skyrim looks like, it needs to have extremely distinct regions. That is the number one reason Morrowind looked bigger than Oblivion, aside from the draw distance.
Definitely.
There should be tundra, mountains, desert, forest, jungle, meadows, plains, lakes, rivers, hills, valleys, etc. The landscape should be diverse, distinct, and spread out. I agree with the people saying that our stereotypes of Norway/Finland do not need to be the influence for Skyrim. This is Nirn we're talking about, not Earth. Flora, fauna, and general geography on Nirn are NOT limited to their earthly counterparts.
I especially hope that they make the sky as vibrant as possible. Sky + weather. At night, the sky should be so vivid and beautiful that I will want to just stop and stare at it as the score swells around me. Oblivion's skies were pretty nice, especially in Shivering Isles, so I have high hopes that they build on that. I hope they improve on the weather though; after playing RDR I know that many of my hopes/ideas for TESV's weather system are indeed possible. Things like puddles that accumulate during rain, massive towering cumulonimbus clouds that you can actually watch roll in before a storm, powerful thunder and lightning that seem to shake the land and show a major light contrast, respectively. Sunrises and sunsets that cause beams of light to reduce vision if you're looking in the sun's direction... Beams of sunlight breaking through distant clouds... Fog, hail, snow that actually accumulates on the ground rather than permanently sitting there.