Ask HN: What's the ideal platform for online discussion?

There's a lot of online communities made for sharing links: MetaFilter, Reddit, HN, Slashdot, and many others. There aren't so many that exist solely for discussing topics -- discussing an issue, rather than commenting on a link about the issue. (Link-based sites often don't have the same depth of back-and-forth discussion, but rather many comments on the article's contents with unrelated points.)

I think this is partly because most discussion forum software is cumbersome and poorly designed, and partly because nobody has built the right community.

If you could build a discussion system from scratch, how would you do it? What would the software look like? The rules? The community?

My current thoughts involve a threaded system that's either invitation-only or has a high bar for membership, although I find current threaded discussion systems unsatisfactory.