I'd broaden the question about sustainability. I haven't worked deeply on OSS, but have worked with a number of other volunteer organizations or efforts, some on my own and some with a community.

In all cases, there needs to be some payback to the folks putting in time. And it can't just be money, because volunteer organizations never have enough money to compensate appropriately. Otherwise they'd be businesses :) .

Other forms of compensation:

   * fun, as you outline
   * recognition from others
   * impact on the world
   * legacy/connecting to the past
   * satisfaction of helping others
   * increased leverage toward other goals
   * learning new skills
   * connection and community