One problem with comment threads on blogs is that they lose the "time-binding" central to human progress: older posts and their comments gradually fade into the background and are, for the most part, forgotten.
A different approach to online discussion is the venerable "bulletin board". There, threads under a given topic migrate to the top of the heap when they are active, and particularly good discussions can be "pinned" so they are always visible. It's still not perfect; lengthy topics are themselves a barrier to newcomers who just want to get the essentials or to oldtimers trying to locate something only partially remembered. But they can certainly be educational and concentrate discussion around a particular topic in a way that's hard to do on an ordinary blog.
For a time there was a bulletin board attached to the ClimateAudit website - it was the main url with a 'phpBB3' appended. It was rather nice, and had a lot of interesting discussion, if somewhat repetitive, but I think it actually proved helpful for certain people. Eli Rabett, Nick Stokes, and I were among the frequent contributors, at least for a time. And then it vanished...
PhysicsForums used to have a spot for discussion of climate as well, but that has been removed in recent months, closed down by the moderator apparently as too contentious. So Chris Ho-Stuart has just organized a new physics-based forum for climate discussions, and seeded it with one excellent tutorial on the central topic of climate sensitivity - I encourage readers and passers-by here to check it out:
Climate Physics - and in particular Estimating the impact of carbon dioxide on climate.