1. Seasons. Autumn would be awesome. This requires some nice scripting and a good engine though, I think.
2. Dynamic shadows as you stated. The world looks very... "blank" without it. It's a must for a new game like Skyrim. I was disappointed when I saw Fallout NV didn't had it, then again... it was on the same engine.
3. Really good looking textures. I hope we won't have to make a new QTP3 for Skyrim. Please make this right from the start.
4. MUCH better water than Oblivion. Real waves, that "splash" against the shorelines.
5. 3D trees, I don't like 2D. 3D handles lighting much better... but if they use godly looking speedtrees, then sure, since it helps with performance. Then again, this depends a lot on the engine. I hope it's a good one
6. Better weather, including lighting. Oblivion's default weather looked very washed out.
6b. "Darker" and "stronger" lighting in general. Oblivion's default weather and lighting looked very washed out.
7. DX 11 support, that would surely help with performance.
8. A subtle depth of field. This is important, and I think they can do it quite easily.
9. Dynamic sky boxes like Red Dead Redemption as you said. Watch this video. Looks awesome, and it shouldn't be too difficult to do either. A mod did this for Fallout NV. http://www.youtube.com/watch?v=aqNJOnhJPFY
10. GODRAYS!
But keep them quite subtle, kinda like Crysis.
11. More unique environment (not really graphics, but still). Oblivion's environment was way too alike... wherever you went. Some variation... high rocks, high hills, low valleys, strange rocks formations, look-out points, big rivers, very small rivers, very deep forests, not very deep forests, real farmlands, etc... the list goes on.
If they manage to do all this, it will become one of the best looking games out there. And if they succeed in gameplay and story as well... oh boy. Then you have a game of the CENTURY!
I believe most of this depends on
not using a Gamebryo engine, seriously... please don't
Bethesda can reach a much higher potential concerncing graphics if I don't.