When I left feedly and came here, I was delighted to find that feeds that I've subscribed to in the past would display an error icon if it 404'd or timed out or was otherwise unavailable. I was also appalled to see just how many of my old feeds were now broken. For when this happens again, I'd like a way to know about it, without necessarily coming to the main site to do it. (I use Reeder more than the main site.)

Naturally, this would have to be done in an unobtrusive and not annoying way. I'm thinking it would be something that users opt-in to use. Either an email if a feed remains broken for too long or perhaps a special feed that notifies the user when a feed breaks. I especially like the feed idea because it would be possible to see it on reading applications and you could, for instance, hide the notification if the feed happens to come back online before the user reads that it's down.