From the site dedicated to their affiliation program:

"XML gives you the power to: Choose which product attributes to display, Control how product attributes appear, Perform multiple product line searches, Build lists from multiple product categories. As this platform proves to be successful, we'll add additional features and attributes to XML documents, such as: Price and availability information, different image sizes, Author and artist, Customer ratings, The ability to specify the number of products and select attributes you want to receive".

It's been available for a few weeks but now they increased exposure to all their "associates". If they indeed expose all the properties of their products, from pricing to ratings, that will come as a strong differentiator versus other affiliate programs.