Bear in mind I'm still pretty new to Oblivion, but I played Fallout 3 extensively, and really enjoyed many of the housing mods availible for it. I really like housing with a background, whether that be a quest required to become the owner of the estate, or even just some books/notes/what-have-you that detail the history of the place. For me, it really helps the immersion factor when I have to work for a home, or if I stumble on to an abandoned home and can learn a bit about the pervious owners, and why it came to be empty.
Even though I voted for Story, I have to say that design, both interior and exterior are important for me as well. I like a home that "fits in" with the world its placed in, and looks put together, but not cluttered or haphazard on the inside.