A comment from Rob Howard (I'll dig out a link later) go me thinking about problems with most blogs. Now, in my every day job I develop community applications, forums and all sorts of similar 'social software systems' one of the big keys to helping discussions to take place it giving a simple path to reach the discussion.
Blogging systems are very different, giving exclusive prominence to the posts, with 'comments discussions' becoming secondary - which frankly is a bit of a shame! So here's the idea, the comments in blogs are essentially single threaded forums (with the initialising message being the original post), what I though I would develop (well once I get my dev machine working properly again...stuipid computers!) is a simple control which can sit in a side-bar displaying the most active discussions. Also some form of notification system, where a user posting comments can optionally choose to have all future replies to the discussion (or simple notification that a reply has been made with a link to the discussion). Problem comes with RSS - would there be any point in providing an 'active discussions' feed, would this even function with RSS readers?

UPDATE: Been having a play with this (the comments tracking stuff) - .TEXT does not make this overly simple (well, not really .TEXT, blogs in general) - the problem boils down to the fact that you're requesting a notificaiton without membership.
Now, there are a couple of ways to do this; since a 'remember me' checkbox exists this could be used as the basis for a rudimentary membership system with the cookie being used as the only authentication mechanism; also, with each notification sent, a URL could be included which would allow the user to 'unsubscribe' from the notification.
I prefer the second option since cookies are just too flaky to be used as an opt-out mechanism (getting deleted, privacy settings etc...). Oddly this is where things like .NET Passport and .NET Alerts come into their own -- and represent a pretty perfect vector for the delivery of such unimportant, transient notifications...so, anyone want to give me a free license for passport and alerts (rough value $20000 :-)), I wish Passport and Alerts were MUCH cheaper, the only sites I've used these systems on are Microsoft's own brand sites and for charities - really a shame, it could be a brilliant system if more developers got interested.
Anyway, I'll look at doing the URL system in the next day or two (just got my dev machine running again so I'm busy installing software right now)