Have a lot of comments on your blog, where discussions tend to be responses to previous commenters? If so, this plugin may help organize the comments a little better by allow for visitors to choose which comment they are replying to, as well as quote a comment.
Looking outside the box, I see a plugin of this nature being a building block for using WP as a forum. If You allow registered users a certain level to start posts, and require registration to comment, each post is the start of a thread, maybe you see where I’m going with this.
Anyway, I can see this being a very handy tool for high traffic, high comment sites.