Discourse is well worth a good look. Their Forum-as-comments integration into WordPress works by embedding only the 5 "best" posts (several factors involved) from a tied in forum thread.

Here's an example post.

Scroll down and click on "continue the discussion" to see the corresponding forum thread.

While we're talking about nested conversations, I gotta say I much prefer Discourse's style of simply including the little "In reply to..." header, keeping the conversation neat and linear. Click it to see some magic happen ;)

Here's a mockup I made. Don't get too caught up in the UI aspects of it (obviously that trailing element would never scale); the point is how I would have liked to structure it:
imgur.com/m6CRLcJ,tfbhQth

p.s. Jeffro one thing that's pretty annoying about your comments form is that it appears to have a basic anti-spam measure that stops me from including links. The problem is that it lets me click "post", and proceeds to refresh the page with my post all gone. Good thing I've made a habit of always copying my posts before posting ^^