Microsoft has extened RSS (RSS, Atom, RDF, etc.) with Simple List Extensions for RSS, which goes a little like this: add extensions to RSS (yes, that’s what RSS was designed to allow, so all you purists, just shake your head and walk away) which allows the RSS aggreator to handle your RSS data in a much richer way than just “email view” (where you see all the articles in a reverse-chronological view, and can’t do much else with them).
In the e-comm world, you probably want to enable your viewers to sort by name, by date, by price, etc., and filter by cetegory, maybe by price, etc. That’s pretty cool. Imagine if CraigsList.org did this, you could filter by Zip-Code and sort by price. With eBay you could subscribe to an RSS “search result” feed and watch the number of bids on, say, a Zune, sort that by price, and watch the process as the bidding occurs, right there in your RSS reader (IE7, for example), and all of this without all the eBay look and feel