I've had this game since it came out, and I have read alot of raves, but I've never quite gotten why it was so popular. I never particularly liked it. I do like how massive the world is, and the fact that there are so many quests and places to go is quite cool, but to me, I'd rather do it with some friends, and since (last I checked anyway. I don't own the expansion...s?) there aren't any LAN capabilities, I just have never had much fun with it. This is the kind of game I would really only enjoy with a large group. Sure it was very open ended, but it was open ended to the point where it was little fun to do by yourself. Well, that's what I thought anyway.
What is your reason for liking it, because it is possible there is something I have missed, or just didn't appeal to me so I never payed it attention.
--edit--
I just thought about how cool it would be to play that game multiplayer. Even a group of 3 would be fun as hell. Damn! I hope they get that in eventually.