Here it goes:
[list]
[*]The environment looks detailed enough to be realistic, Oblivion's environments contained only bushes, trees and rocks (and not a variety of them either).
[*]The weather gives an amazing feeling, unlike Oblivion where it was generic.
[*]3rd person animations and models look AWESOME! Everything is fluent, and with the new animations the world looks so much more alive!
[*]At LAST, one of the most important features I personally felt a lack of in Oblivion: EPIC battles! Dynamic battles! Different types of battles!
*Plus, if you noticed, they did actually slow the fighting down so you have more time to make decisions. You can't just run backwards and dodge everything. Every cut actually felt like a cut!
[*]The dragons actually PICKED UP A GIANT! AND AFTERWARDS ATE AND THREW AWAY THE BODY OF A GUARD!

There are WAAAY more reasons, but I believe this will do for now.