Yes, the MUSIC. The harmony of the orchestra simply catches the atmosphere of every moment, from exploring the wilderness to gazing at the night sky. It's like nothing I've ever experienced in a game.
Bethesda = Da Vinci/Chopin of game development.
This, and yet you only told so little. I'm not amazed that popular culture feels that "epic" must be delivered "more epicly" all the time. This is the case with Skyrim's music compared to Oblivion as well. Yet, there are SO MANY KINDS of music in Skyrim which TES games never did! There are politically infuenced tavern drinking songs, steady violins just to call forth the following epic unleash, a Morrowind music comeback, some silent minimalistic music in quiet situations, etc., etc., etc. Just perfect music for every situations, and - is it? - even different in different towns, etc. Also there are very high quality, fitting and loud sound effects everywhere to boost every single aspect of the music and overally the world.
I think the music and sound effects are so good they just blend in the overall feeling of the game; you cannot often even notice how variable and fitting they are until you pay close attention. This is definitely a huge part of why I call the game's vibes magical.