The "forum system that is most conducive" to our goals might well be a combination of one very open central site (LessWrong itself) supplemented by a variety of more private sites that discuss rationality in different ways, catering to a variety of niches. Not just Eliezer's Facebook page, but including things like MoreRight, Yvain's blog, Overcoming Bias, Give Well etc.
This makes me a little suspicious as a solution, only because there doesn't seem to be anything particularly special about it besides being precisely the system that is already in place.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
Notes for future OT posters:
1. Please add the 'open_thread' tag.
2. Check if there is an active Open Thread before posting a new one. (Immediately before; refresh the list-of-threads page before posting.)
3. Open Threads should be posted in Discussion, and not Main.
4. Open Threads should start on Monday, and end on Sunday.