chromatic from SlashDot has some great points about what constitutes an online community in his article. Let me dot-point:

Exist For a Reason (what I call ‘compelling content’)
Users Draw Other Users ( ‘viral marketing’)
Users Will Surprise You (and astound and amaze and inspire )
A Sense of Ownership
A Shared History and Culture
The Bell Curves
Barriers Are Mixed Blessings
The First Contribution (wow what an important point!)
The Interface
Mischief
Discuss the Community
Openly
Don’t Stop There!

The article is another old one (2002) and sponsored by O’Reilly Networks (love those books) but once again relevant and interesting.