Looking at the demo page, I love how the threads are 1D, but you can still switch to direct replies if you just want to follow the sub-conversation. It's the best of both worlds.
Which brings up an interesting thought: why does a discussion have to be between two individuals? If we are all reading the two current sides to a discussion, are we not all entitled to share our opinion to continue the discussion? It's then a group discussion.
* It is harder to build mental model of opposing view as the number of people on the opposing side increases do to inconsistances in their view points, experience, and reasons.
* It can be harder to judge how much an influence your arguments have the more people who are participating.
* Reinforcement with a peer group. There can be a tendency to not examining a topic closely if there is a peer group that shares the opinion and is holding strong in their opinion.
There are many advantages to one on one conversations that are lost in group discussions. I would love to retain the benefits of one on one conversations but on a forum involving N people. I do not think there is an easy solution though.
Example: HN in that setting would be just one node, called Hacker News. Each submission would be a post/comment and all the comments from here would be just subcomments under that parent post. If someone posted here about new language for example, that post along with all discussion under it can be become a new "forum". If you move it away from under HN, you can still leave a hardlink there so it won't be lost.
Is the discussion there civilized though? Mostly not.
edit; ah, it has that - http://try.discourse.org/t/yo-dawg-heard-you-like-replies-so....
I like the new UX ideas. I'm wondering if instances will be able to federate, and the 'front page term[/hashtag](s)' for the discussion area of a certain website on a topic could be framed differently.
Some topics are going to get to crazy posting speed. Watching Reddit posts with new set on isn't that fun. An IRC/Jabber MUC format would be good for times, and Facebook autoupdates its newsfeed anyway. An IM system with text (and at least url) post and their replies upvoted, and maybe filtered in some tiling window like fashion?
There's my half-baked idea for the evening!