Your nascent developer community needs to have something runnable and testable to play with. When you start community-building, what you need to be able to present is a plausible promise. Your program doesn't have to work particularly well. It can be crude, buggy, incomplete, and poorly documented. A happy programmer is one who is neither underutilized nor weighed down with ill-formulated goals and stressful process friction.
Enjoyment predicts efficiency.
It may well turn out that one of the most important effects of open source's success will be to teach us that play is the most economically efficient mode of creative work. If you treat your beta-testers as if they're your most valuable resource, they will respond by becoming your most valuable resource. The next best thing to having good ideas is recognizing good ideas from your users. Sometimes the latter is better. Free software is the leading edge of what is to come, the first product of the indigenous culture of cyberspace.