I just read the whole thread, and I found Weston's ideas particularly interesting. It is a danger that the private forum becomes "the place to post" and that none of its members care about the regular forum anymore, which would just host a pile of rubbish threads. But I feel that this is also a matter of personal motivation from the private members : sure it would be easy to fall into that scheme, but if they are aware of it and pay a little attention (keep reading and answering regular threads as well) I don't see why it can't be just fine.
As for new members not being allowed in because they can't prove their value on the bad threads, I think this can be easily avoided if they can apply through a form. I don't think post-count and participation on the forum should matter so much for the form, as they are new and obviously didn't post a lot, but it could be some kind of "motivational letter". That way they can show what they are worth and be allowed in the private section although they are new.
This might also make the separation between the "good" and "bad" thread worse, as those interesting-people-but-not-in-the-private-section-yet might have helped keeping the good thread level up in the public section.
anyway... Except for those problems that have been pointed out, I think it's a good idea. One of the reasons I almost never post anything is that I find it pointless : most discussions don't interest me that much ("would you stop cubing for a girl ?", "Look a new crazy good F-perm... ah, no it's already well known", "hey, I'm selling my cubes !") and I don't want to add some more random blablablabla on top of that.
So I'm actually really curious to see how that private section will evolve and I'm looking forward to read some more constructive thread !