I have repeatedly seen such communities become places where folks ask totally unrelated questions because the community itself is such a good source of awesome ideas and info that they prefer to ask there, where it is "off topic", rather than asking some other board they belong to where it is "on topic" because the quality of answers is generally so superior. So I think that it depends in part on how it is moderated. I think it is really difficult to intentionally create a community with the kind of goal you describe. There are lots of challenges in terms of trying to find a common theme to bring like-minded individuals together without so narrowly defining it as to put a straight-jacket on the conversation. Quite a lot of the forums I have belonged to wrestle with such issues.

An example I can think of, which might not make sense to you, is that "gifted homeschooling" groups tend to be ripe, when handled right, for the kind of atmosphere you describe. But then you cannot discuss "just anything" -- some folks will be very sensitive about what topics get brought up and parents are inherently more conservative than other groups. I think this is true because "gifted homeschoolers" are frequently dealing with 2xE kids -- kids who have issues and just don't fit in -- yet the emphasis is on two positives rather than on the handicaps and other negatives. It's tough to package a community for dealing with serious issues without it becoming a place that reinforces the negative self-image that is typically bound up in such problems. I think the idea itself of "gifted homeschooling" forums is a neat hack that gets around that issue. So far, I feel I have failed to adequately replicate it in trying to reach out to others in a positive way for various serious problems. :-/