I would say an ideal forum would be one with lax moderation. Of course ban/infract members who are just dicking around and purposely insulting members, but if a message seems offtopic, don't be so quick to delete it. Posting in a forum should be like real life conversation, only organized. Instead of having topics come to you, you can in fact choose what topics to discuss. And just like any real life conversation, some posts may be considered offtopic because they stretch out onto a tangent. That should be fine.
The staff needs to be serious about its job. A lot of places that require positions of power without any actual compensation for responsibility results in carelessness and a corrupted (to put it extremely) staff that goes against its own rules behind the backs of regular members and ultimately try to improve the forum for what best suits their personal interests before the ones of its members. The staff needs to be built of people dedicated and passionate about the forum's main topic (in this case Pokemon and Pokemon-related events) and keep the forum interesting. The beauty of forums especially is that it's the community that provides its own entertainment: the staff can sit back and let the members interact via various topics within various subjects. A good example of proactivity by the staff would be the PokeCommunity's annual Get-Together event.
I think a good forum is one with a large memberbase. More members means more topics, more posts, more thoughts, more insights. More members means the demand for more to be done or to be included. Our Culture & Media forum, for example, is fairly new to the forum when you consider how long PC's actually been running. Only with a large community will things progress and improve. A small, tight-knit community may want more to be done, but not as much or as in high demand as a large group of people. And it's within these large communities that smaller groups form through similar interests.
This should be PotW.