Actually, in my opinion a members only forum could be a very useful asset. While I have nothing against a certain amount of transparency, there are some topics that are not appropriate for the those that aren't familiar with the site. Topics, that unless one knows the way the site works, are really open to misinterpretation and misunderstanding which could cause some potential members from joining.
I don't think it would be attempting to deceive anyone and lure them into becoming members under false pretenses. Frankly, discussion about policies or changes we would like to see made to the site are not a concern for non-members until they actually are adopted by the site. I would consider it more likely to be seen as an attempt to draw in unsuspecting potential members to have them read about all kinds of features some members want but that haven't even been looked into by site admin. And when it comes to airing dirty linen, I don't see where it is appropriate to publicize all our personal feelings of the site's shortcomings to everyone that happens to visit the forum.
Personally, there are several topics that I think should be discussed amongst the members only, such as; the "Make Offer" option issue, miss categorized listings, listings & stores abandoned by sellers, etc. These and other issues, while of legitimate interest to members, doesn't, in my opinion, have any business being opened up to the non-member portion of the public. After all, most businesses don't stress there products flaws in their advertising.