Funny. I found I needed to go track this down just after you did, to quote a passage for my 
workgroup.

I suggest you start with <http://zesty.ca/roundup.html>. Then you can follow its links to the 
first and second round draft specs, which seem to me to be fairly well represented by the current 
design document. But this earlier document is the most concise description I've seen of what 
makes Roundup stand out as it does.