Click Marketplace, then click Channels, and then look for a channel called Rock Picks Just For You.
Zune takes what you've listened to, compares it to people that listen to what you listen to, and recommends songs that those people listen to, on top of what you listen to. You can subscribe to this, and it auto-updates.
If that's a bit confusing, let's say you listen to Muse. Zune knows that you listen to Muse, and it looks for other people that listen to Muse. Zune then looks at the other people that listen to Muse, looks at what else they listen to, and recommends that to you. Like someone who listens to Muse and Paramore, Zune takes the Paramore songs they listen to the most, and tells you to listen to them.
Hope it helps.