Having been the one to ask the original question, my own resonse is actually that I'm not sure. What I had envisaged was probably something casual along the lines of the Double Bar (which, incidentally, I haven't ever looked at much) but which was only accessible to long-standing posters, who will inevitably have got to know each other's characters and opinions fairly well, and who will not easily offend each other by saying things like, "Catch a grip, dear, where on earth did you come up with that idea in that thread?"

I know that PMs exist for this type of thing, but a PM is only a one-to-one discussion.
And sometimes I get the feeling that serious posters with valid arguments back out of a thread because their arguments are simply not taken at face value, but, rather, are taken as offensive, and so the whole thread gets bogged down in these people defending themselves against the easily offended, or backing off. Or else the whole thing goes to the dogs because of a few ridiculous comments which it takes the next two pages to be dealt with, at which stage the crux of the matter is lost! I think Rosemary might have a point in suggesting that maybe it would be at this point that the mods could take the decision to move the thread to the passworded section, where the discussion could continue without moderation - within reason, obvously - and without hyjacking. Maybe a combination of your username and a pin number could get you there? 'Adults Only' was probably a bad choice of expression, as I agree with others that age 18 is not always relevant.
Just a little food for thought! It could, in reality, create more problems than it would solve, but I was interested to hear others' thoughts on the idea!