Plain and simple:
Exploration. And I'm looking at this from a playing on Xbox 360 perspective, so vanilla game(and I haven't had the money for the expansions).
Even if I did have a PC that would run it, I wouldn't count mods, because a game has to stand up on what it is in the beginning, not what people can do by tweaking, tinkering, changing the game after.
Seriously, I've gotten lost in Skyrim for over 120 hours exploring and doing just the non-storyline quests. I'll admit that I haven't even beat the main story yet, not really that far into it either, only just completed Diplomatic Immunity yesterday.
The land is vast and it just looks so good, even on the less graphically inclined console. I've discovered and been to over 150 different places and I know I still haven't seen nearly half of what is in the game.
Also I don't just encounter new things in the new places I find, with the randomness of the game I can walk around places I've already been and still see new things randomly happening, random npcs going wherever they are going, or getting into fights with other npc or creatures without me being close or sometimes even without me seeing it because I got their well after the situation occurred.
Lastly, who doesn't love random dragon attacks at any time to keep you on your toes, even during times when you are in the middle of a quest that doesn't even involve dragons. Example: I was doing that Daedra quest with the talking dog, and as I was following the dog to where I needed to go to do the rest of the quest, a random "ancient" dragon came down and I fought it along with the help of said talking dog. Fighting dragons with a talking dog, sounds like some crazy new fantasy/comedy show for TV.