You have to look at the community from it's member's side. Take your organisation out of it, and answer "why would people join this community?".
The purpose of your online community should fit into one of these 4 categories.
Leisure: Some people spend their leisure time in online communities. People come together, talk about games, sports, TV, music and have fun doing it.
Relationships: The world is a lonely place, it's hard to approach a strangers. You might try to find fellow recovering alcoholics, expand your business network or be seeking true love.
Fix Something: Something isn't right in the world and you want to fix it. Maybe the environment is going to hell, your politician is doing a bad job, or Microsoft Vista isn't as good as it needs to be.
Self-Improvement: You want to improve your life. Perhaps be better at your job or expand your reputation? Maybe it's to save time or seek help in finding the perfect pair of shoes?
Perhaps the very best communities can fit into all four. However, if your community doesn't fit into one of these, it's going to be much harder to build.