Scenario:
"I have an unusual URL or blog domain set-up, am using multiple domains and/or subdomains, and/or using multiple blogs. How come the links in my RSS feed aren't linking properly?"

Solution:
Your problem may be related to having blogs in subdomains or having an unusual blog URL defined. In most cases, you can fix this problem by going to the RSS template (Admin area > Templates > feeds/rss20) and finding this bit of code:

<link><%blogurl%>index.php?itemid=<%itemid%></link>

and changing it to:

<link><%blogurl%>?itemid=<%itemid%></link>