Dare Obasanjo observes that there's a couple of equivalent's to RSS' <enclosure> element; <link> and <content src="...">. The 3rd gotcha I found was content with a type other than inline text or XHTML. It's legal to have content that's pretty much anything; an image, an MPEG, whatever, but aggregator authors are going to need to decide what to do with it. The closest analogue I could come up with is attachments in SMTP or NNTP, but I believe that most aggregators don't include that concept yet.