You get basic functionality (private messages, mimicking "Wall" functionality calling it a "space," commenting on items/posts,) and can follow friends and artists. The space should show your friend's purchases (unless you check a "do not broadcast this purchase" box.) You should also be able to do public recommendations that show on all of your friend's spaces. "Jeff Warren recommends DJ Z-Trip's song "Kiss (Remix ft. Murs)!"
Every item you look at should include general ratings, friends ratings, and a list of friends (and artists you follow) who've suggested it.
As an artist, you can put up music, video, HQ images. They should undergo a cursory inspection (I'm sure there's software to match audio against known audio,) and then be uploaded to sell.
Bonus points if you want to do an "online library" akin to Amazon's Cloud Player. I'd also let you regulate people's ability to see your library (everyone, friends, no-on) on a per-item basis. Hide those guilty pleasure!
Emphasize what Facebook lacks: - The ability to compartmentalize your networks according to personas rather than "lists" (i.e., different profile photos for different groups of people, etc)
- Have the service be an arbitrator of sorts between oauth providers
More than anything:
DO NOT TOUCH MUSIC
Let it go for now.
I wouldn't do anything to it until I was sure I had that figured out.