So, the big concern with a private forum seems to be that people feel there will be valuable information present in such forums that they may no longer have access to. Correct?
Solutions:
1. private forums are publicly visible; read only.
2. private forums are absolutely private, but after an expiry time they become public. Extensions on the time could be requested.
3. one or a combination of the above where the owner of a thread can toggle the thread between private and public until a public post occurs. Such threads would be flagged as not permanently private so users could decide ahead of time whether to participate or in what fashion.
However, I find focusing on the idea that information will be kept from you, and that you would consider this information relevant to you, seems to lack recognition or acceptance that the system already does that, that people have the right to determine that for themselves, and that it is simply a tool like all other functionality that has the ability to be used in a constructive or destructive manner. I have private discussions with people through DB private messaing, in person, in emails, and in other forums. So, am I harming the community by doing so? Is it detrimental? Sometimes we wish to flush something out before we commit it to public. Sometimes we want to interact with our close circle only. Do you have a right to know that I am going to go train at the pool with my friends doing apnea until we each samba? (not true but it is an example) Do you have a right to know that I am going for a world record and the discussions with my close friends about technique, strategy, etc? Maybe once my record is over I will toggle the whole discussion for public, but there was a value to me at the time to have it private.
I think it would be easy to implement private discussions that can be toggled public and therefore only threads that the participants saw a value in being kept private, would, and I believe people deserve the right to make such decisions. As a matter of fact you can't stop them from doing so. So, is it more constructive to push them a way from a resource that inspires exchange because they have the other need? The point is the private discussions will occur no matter what. So, the real question is what is the balance of gain vs. loss by explicitly supporting them?
It looks to me that the fear of being cut off is clouding our vision of what is likely. There is definitely implicit gain in having private discussion support. I am sure we can all agree on that. So, is there a real loss if this support were added? Well above I state the obvious that private discussions will persist with or without this functionality, therefore by adding it, we are not suddenly creating the new possibility of private discussions. Therefore, the only argument left is does private functionality inherently promote a loss of relevant information from the public forums. I don't see this as likely especially if some of the features I suggested above are employed and expanded upon. Most posts would be initiated in the public forum unless the owner wanted to have a private discussion with his circle of acquaintances. If as they go a number of them sense that this would be in any way relevant to a general audience and does not affect their need for privacy, then they release it to the public. Simple. Everybody is interacting in one integrated environment and everybody's happy.
No?