Pretty much all of thee above, but I voted for the music because it's that damn good.
As good as Oblivions was, by this time playing it I had already switched to a custom soundtrack. (re constructed ancient Greek and Roman music, and sometimes "Subbatum" which is Black Sabbath covers played and arranged as traditional medieval music, bad ass. Think The Omen meets War Pigs

)
Skyrim's has the perfect epic ambiance without ever being intrusive about. Some of the instrumental pub music is as good as anything in that sort of genre I've heard.