Mine has to be the environment such as the mountain path
Or the new skill interface:
What about you? What is your favourite part of Skyrim so far?
My favourite thing about it is that it comes out in 6 months!
Probably the radiant storytelling.
I like the idea of talking to someone about a quest you were doing and them having done something similar but at the same time quite different. Also for replaying it will be good as things will play out differently.