A few months back I floated the idea of having an option to be notified if someone replies to a comment on a blog entry. At that time some said "This is not a forum", etc... But I still think this is an interesting feature. I might introduce it with a configuration switch so each blogger can turn it on or off.
Leaving it on will add the advantage of a higher traffic to your blog, bringing people who is really interested in the conversation back to keep it flowing.
Comments? ideas?