Since I'm working on other kinds of software right now, and the RSS world is more fluid than it's been in a long time, it seems it might be a good time to share some things I learned in the first go-around.
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).
OPML should be a file type you can subscribe to. That makes it possible for the user to connect their account on the central subscription server to your aggregator. Also, this feature opens the possibility of a sort of super-curator, someone who manages a collections of feeds that people can subscribe to in a unit.
Also in the argument betw mailbox and river type readers, why not do both? The same data can be viewed either way. Some times you may just have a few minutes to quickly skim the news, and other times you may be able to read feeds individually at a slower pace. I did something like this in the LBBS software in the early 80s. There was a river, and a hierarchic view. You could switch instantly between the two views, and stay on the same item. The same approach would work well for RSS (though I've never seen it done, if I were doing a new reader, I'd definitely give it a try).
Also a feed should have a way to tell readers "I'm done" -- no more updates, stop reading me. That would make it possible to do more things with RSS. I don't want to subscribe to a special feed about a specific event, knowing I'll have to remember to unsub. Better to let the feed tell the reader. This feature is documented in my microblog namespace and it's supported in my Radio2 linkblogging tool.
There are lots of other ideas like this. I'll write them up as I remember them.