I'm traveling at the moment with limited ability to participate here this week. I'm also a long-term member of the C&C group that Roel and Minniev are referring to. The weekly discipline of submitting a photo, considering the opinions of others and honing my own analytical skills has been invaluable.
Getting the "pitch" of discussions right is vital. Especially with newer participants and those just beginning their photographic journey, honesty and encouragement is important but it must be honest and it must be analytical. "Great photo. That's the cutest cat." is not analytical.
The best assett is a group of people who are experienced in doing this because they can set a, for want of a better word,"tone". The format of the forum is critical. The threaded view format at the previous site worked well in making discussions easy to follow and link to the image. It is going to take some time to get a new satisfactory format in place but the outcomes are worth it.