I think HN teaches some great lessons. Building an online community has little to do with the technology of the platform. What matters the most is a strong robust set of founders, slow growth, and vigilent moderators. The founders will set the culture, and slow growth will help new comers to adapt. If you experience rapid growth you run the risk of new comers bringing in their own cultures without experiencing some of the "we don't do that here forces" thus diluting what makes the community special. Diversity of backgrounds is great, but once they're here... there are sets of expecations about behavior.