The main thing we learned
is that subscription needs to be centralized to make the process as simple as possible for the user. That's one of the main reasons Twitter was such an effective competitor. The central server keeps an OPML file for each user. Each site with a feed links to that site with the URL of its feed as a parameter. When the user clicks a Subscribe link, a dialog appears confirming that you want to subscribe. If you say OK that's it, nothing more to do. The user never sees a URL. No browser has to cooperate (not that they would).