Have you seen AudioScrobbler? It's a webapp that keeps track of all the music I've listened to. Combined with a social network, it purports to recommend music and do lots of other fun stuff. While a wicked cool idea, it seems like they're struggling to scale it, and I've never actually gotten a recommendation out of their system. (though I do drop them a few bucks each month because I dig what they're doing).
You should build this functionality into iTunes, and automatically create
users.itunes.comaccounts for all of us iTMS purchasers. Though I love what the Scrobbler folks are doing, I think you could do it better.
You could recommend artists based on what I've purchased and been listening to. You could let me keep a friends list and let me know what my friends have been listening to, like AllConsuming does for books.
You could load the whole site up with feeds like the Flickr folks have done, so I could get updates each week in my aggregator. And the URLs could be nicely rewritten:
/artists- artists I've been listening to recently
/albums- albums I've been...
/genres- genres I've been...
/ratings- aggregated ratings data from my iTunes
/playlists- playlists I've created in iTunes
/imixes- iMixes I've created on the iTMS
/wishlist- songs & albums I want to buy
/stats- statistics of my listening habits
Monetizing the site shouldn't be too tough... ;)
Can't wait to see it!
Update: Tags in iTunes are a must-have, and here are two music recommendation sites: MusicMobs & Goombah.