Terry--I'm a member of the Community Roundtable, a peer network of community managers, and it's an interesting mix of both a small online community and a quasi-association. As someone who works in the association world, I'm used to associations that exist primarly in "real" life, with online community being a very distant after-thought, if it's a component of membership at all. With the Community Roundtable, the online community IS the association, for the most part--there is no annual meeting or other face-to-face component. As such, it's much like you describe--specific discussions with a big networking component.
The thing I like about it is that the community managers ensure that questions posted get seen by people who likely have an answer or have expertise in that area. In a huge online community, there's no way that level of personal attention and care could scale; in a small community, it's possible for a community manager to see a question that's gone un-answered and reach out individually or through a weekly email to make sure the question gets answered.
While large numbers seem to be a metric of success for a lot of marketers and community managers, I agree with you that there's a lot of value to be had from small, intimate online communities.