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?