Bernard, thanks for restarting this. If you hadn't I was thinking of doing it. As you know, I am in favour of this idea.
To quickly summarize my previous contributions and some thoughts of others I retained:
(1) We only need very simple guidelines: keep the discussion constructive, no insults, do not personalize it, and stick to the artistic and technical aspects of the photographs.
(2) Within those limits, it is open discussion between photographers, so nothing prevents the person submitting the work from responding to comments of others.
(3) Submittors are free to accompany their photographs with some commentary if they think it useful, but in general one expects photographs to speak for themselves.
(4) It isn't a mutual admiration society - people will say what they think subject to (1).
(5) Given the above, the forum will take on its own character as determined by the participants. Participants will self-select accordingly.
(6) There should be a moderator with a light hand in case something goes out of control and deserves intervention.
(7) With respect to format, it can start as simply as the recent discussion of Lisa's image: it is posted within the Board format and comments simply follow. This takes no particular administrative effort - perhaps only specification of space constraints. If the whole initiative has traction, the administrators may want to think of turning it into something a bit fancier with nicer display options, etc. But not yet.
Hope this helps.