Other: an interesting world to explore with a unique sense of culture. Morrowind nailed it, Oblivion failed it.
This, but I didn't vote other, I voted giant mushrooms. It was great to travel around in Morrowind, never really knowing what will come up. Same with reading books, learning new things etc. Oblivion had parts of that feeling, but it became quite predictable after a while. It started to feel as if there was nothing new and quirky to do, it would all just follow the same pattern.
Making interesting, fancy terrain, creating interesting lore, emphasizing on what makes Skyrim so different from any other thing we know in as many ways as possible, keeps the game fresh and makes sandbox-games fun. Here's to hoping Skyrim will be like that (and that mushrooms will not be forgotten). :thumbsup: