Let's brainstorm user experience some...
The community interaction aspect of the system points towards a web based interface. But the iTunes like seamless interaction would probably require a thick client for the desktop.
Perhaps a combination of the two... a thick client that manages your downloaded music and schedules new downloads....
ooohhhhhh.... maybe the desktop client could be a podcast tool. oh oh oh, I like that idea. Part of the service could be to subscribe to "podcasts" of recommended music. Every night your desktop app automatically downloads the latest songs that are recommended for you.
With all the podcasting hubub, I'm actually a little concerned about making that idea public... but oh well.
Ok, so let's describe the user experience for that.
You browse around the site, and download some songs you think you'll like (based on comments and reviews). You like some of the songs, and add them to your personal genre. After enough time you've built up a big enough list of rated music that the system can begin guessing at your tastes.
At this point you install the desktop app that watches the service for songs to downloaded based on recommendations.
If Adam Curry has taught us anything though, it's that podcasts use a ton of bandwidth. Perhaps the thick client can use an implementation of bittorrent to distribute the files.
This actually ties into keeping subscribers around. Imagine if after rating enough netflix movies, extra movies just ended up in your mailbox because netflix thought you would like it. And if you don't, then you can just send it back, at no extra cost to you.