I took a week off from reading blogs last week (as is evidenced by my responding to week-old posts). I ended up having about 600-700 unread posts in RssBandit.
The UI took care of it pretty well, since I have my opml separated by category I could zip thru most posts and decide whether or not I wanted to keep it around in the aggregate view. It got me thinking about how this would scale for a web app, tho. I don't remember how Bloglines handles this (don't you just get what's available at the time if there are unread feeds available?) but it seems like an interesting problem: how do you keep a rich experience in a web app without sacrificing performance or content that I may want to read (mark all read is not always optimal for me (I always feel like I'm missing something golden (I did subscribe to the feed, after all)))?