For starters, you'll be able to log in to your Cydia/Rock account via your Facebook or Google account. You will also be able to back up any packages, and reinstall them on another iOS device (i.e. your iPod Touch or new iPhone 4). Your previous Rock apps will also be transferred to your Cydia account so you can download them without paying again.
While it's not certain, there might be a rating feature as well, letting customers provide feedback on available apps. There is no release date to speak of, but we'll keep you posted.
source: Redmond Pie