Seeding is critical. You need to have the ability to reach out to the thought leaders in the community and have them participate early on. Quora is a great example of this. Their early community was of a very high quality which is why the answers to many Quora questions are more intelligent than you see on other Q & A sites.
Never forget that the centre of gravity of a community is set very early on. And once that centre of gravity is set it is very difficult to move it. The reason that most communities never take off is that the centre of gravity gets set in the wrong place and the community becomes too negative, too mainstream, etc. Your first 1,000 people in a community set the tone that will be carried through the remainder of the community to be very conscientious as to who those people are.
Also, make sure to take a zero-tolerance approach to spam, off-topic, etc. in the early days. Any instances of time-wasting crap in an early community are like a flashing red light to people checking out the community for the first time. As the adage goes, you never get a second chance to make a first impression. Make sure to minimize or eliminate the chance that a first-time visitor has a negative experience.