In a letter Marc Whitten has confirmed the the Games Marketplace will be getting a user ratings system this summer.
This is what he had to say:
“User Ratings: We’re adding a five-star rating system that lets you easily rate any content in the Games Marketplace. Now you’ll be able to show your support for your favorite games and Game Add-ons on Xbox LIVE and also quickly organize by user rating to find the community’s favorite content. This has been one of your most-requested features, and we’re very happy to bring it to you this summer. “