Online, there are a few things needed to make a community succeed.  Great technology of course but I think the #1 thing is a lightning rod.
 
This was really driven home in this presentation looking at online communities.  There needs to be one person, or one personality who keeps the community growing.  It is not different than in the real world, where a rabbi or minister leads their congregation. 

They serve as community leaders and facilitators - they do not necessarily drive an agenda, and are certainly not "in charge" of the community.  But they are the type of personality that reflects ideas, that makes you want to be part of what they are doing, and gets excited by responding to questions and being the most active member.  Some great online community lightning rods I have come across:

Dries Buytaert - the founder of Drupal.  He has conceived of both great technology and a gigantic community of open source developers to contribute and share and grow the idea. 

Bob Parsons - not my cup of tea, but his blog is a great example of using a company blog to bring a personality and face to an organization.  He is certainly the lightning rod in the hosting industry at the moment.

Tom - the founder of MySpace.  He is almost always online, always facilitating the growth of MySpace.  It has obviously grown to have its own voice - but you always feel like Tom is watching.

First three that came to mind, but there are lots more out there on the big scale... perhaps Barack Obama's new site comes to mind also.  But what about this idea on a small scale?