Geek & Mild by Sean Sperte

Michael Mistretta explores how genius the new iTunes Genius feature really is:

Apple could easily add an average rating to every single track and album, based on millions of samples. Even more powerful, they can show only average ratings based on users with similar musical tastes as yours, further targeting your specific preferences.

I think they’re already doing this transparently with the Genius Sidebar. It’s my understanding that the recommendations are based on ratings of users with similar tastes (among other data).