The brochure for a KM World conference contains a nutshell description of a session on this topic. The speaker’s blurb includes eight key themes that I quote below and comment on.
“communities should be independent of organization structure” – the topic or goal holds the community together, not reporting lines or functional silos;
 “they’re different from teams” – knowledge development and dissemination motivates communities and they can last for years, teams can simply decide and disband;
 “they are not sites, blogs or wiki’s” – technology enables communities but personal contact nurtures them;
 “community leadership and membership should be voluntary” – if a community member is dragooned, their contribution will be nominal;
 “communities span boundaries” – see  above;
 “communities need a critical mass of members” – if only one or two worker bees make honey the community will not buzz; the hive must sustain the vitality of the group;
 “start with as broad a scope as is reasonable” – to me, this point is less compelling than the others. I would not presume that huge boundaries are a recipe for success; and
 “communities need to be actively nurtured” – the general counsel needs to encourage communities, recognize them, and invest in their prosperity (See my post of Feb. 16, 2010: centers of excellence with 6 references.).