Just a quick FYI, make sure you don't use an ampersand "&" in your community name (control panel) it will cause your xml feed to contain errors which will prevent parsing.
or just make it & and have xhtml validity in all forum pages
what is the difference between the <link> and <guid> tags?
The guid tag uniquely identifies an item. For example, if a resource is published at a particular URL on one day but is archived at a different URL, the guid provides an aggregator with a way to link into a resource at its constant URL. Similarly, if a resource is edited, RSS aggregators that use the guid tag are smart enough not to redisplay the whole item to a reader.[/]
That's what I could find. guid is an id, where link is just the url