My Favorite new feature is the Perk Tree system. Making Leveling up meaningful in The Elder Scrolls games was something Oblivion Lacked, and Morrowind had for kind of the wrong reasons (Things like Guild Advancement).
I don't like to take the bad without the good, or vice versa.
So the Feature I'm most wary of, is the Radiant Story system. It seems great, but I hope it isn't used as a justifiable form of laziness.
What are you worried about? Radiant Story doesn't create the stories and quests for you to do, it simply tailors them to you with certain characters, places, and creatures based on your stats. I'd say that will make every quest feel that much more personal and rewarding. I'm not worried about it in the slightest. I would have to say it seems to be one of the most brilliant advances in tech for any game I know of, and especially for TES.
My main favorites would be all of them. It would be far easier to decide on that which I DON'T like than that which I DO like.