Something I've wanted since the day the "Responses" tab was added: An RSS feed for replies to my questions/answers..
There's feeds for about everything else, but not this.. The obvious problem is authentication with OpenID, since you need to be logged into access the feed.
One possible solution to this is, protect the feed URL with a unique token. This is how many private torrent trackers resolve authenticated feeds. Basically the RSS feed address would be something like
http://example.com/users/recent/0/recent.rss?token=94ff6f5039d60ea569182dcd3e64df7c and the user would have a "regenerate token" button should they wish to invalidate the old feed address.
(the email responses option isn't what I'm looking for, and routing the emails through an email->RSS system seems kind of.. backwards)