The biggest motivation is not the money but the impact.
I believe that software, and in fact entire companies, should be run in a way that assumes that the sum of the talent of people outside your walls is greater than the sum of the few you have inside. None of us are as smart as all of us.
Technology is best when it brings people together.
Akismet started on a $70 dollar-a-month server. Anyone can scrape together $70.
Simplicity not simplistic.
I'm an investor in MakerBot, which is a good example of the 'thingiverse'. The idea of applying collaboration and rapid iteration to things that we interact with and hold in our hands everyday is super revolutionary.