After reading up on the CommentAPI, if I were to write the RatingAPI, I’d pretty much plagiarize the CommentAPI but only make the following changes:

Remove the title and link elements (they are unnecessary).

Change the html discovery to:

<link rel="service.rating" type="text/xml" href="url goes here"
title="Rating Interface" min="0" max="5"/>

Change the new comment element to a rating element:

<wfw:rating xmlns:wfw="http://wellformedweb.org/RatingAPI/">

    <wfw:endpoint>http://bitworking.org/news/ratings/52</wfw:endpoint>
     <wfw:min>1</wfw:min>
     <wfw:max>5</wfw:max>
 </wfw:rating>

What do you think?