Another issue is that top-level posts have a feeling of permanence to them. It's like publishing something. I'd rather start with an idea and be able to discuss it and shape it. Top-level posts seem like they should have been able to be exposed to feedback before being judged ready to publish. I'm not really sure what kind of structure would work for this, but if I did, I probably would have jumped into an open thread or a meta thread before now :)

Google Wave is decent for this - it's wikilike in that document at hand can be edited by any participant, and bloglike in that comments (including threaded comments) can be added underneath the starting blip. There's a way to set it up so that members of a google group can be given access to a wave automatically, which would be convenient.

I have a few invitations left for Wave, if anyone would like to try it. I'm not interested in taking charge of a google group, though.