A few things I can think of right now;
- A single good idea - focus on building around a niche community, whether it's a special interest, or a geographical place giving the people a single idea around which to rally is central to your cause.
- Keep them busy - I'd say a small community of active engaged users is bigger that a massive one of disengaged people, having a constant flow of valuable content and active members to engage with is going to give people a reason to keep coming back.
- Ease of use - making sure that people can get in and around your site. That registration is seamless and people don't get lost using obscure superfluous features.