I find that motivation in the open source world generally comes from scratching an itch. I'm motivated to work with a project if I use it regularly, if it does something particularly useful for me, if there's a shortcoming I can see my way to fix.
The hard part is finding not just like-minded talented people, but specifically those for whom it scratches a similar itch: I think those are really your target "market" of people who might join in. In the commercial world, we employ sales and marketing people to go out and persuade people that our software scratches their itch, but we don't usually expect to find people to work with us. In the free world, the measure of credibility is different and I don't know of any better way than what you're already doing:
- Exist
- Be of quality (waves hands - whatever that really means from a fellow geek's point of view)
- Build up a user base (which is a double-edged sword, as you've found, with regard to bug reports)
- Make sure you're googleable so that somebody with the same itch and enough nous to search will find you rather than reinvent the wheel for themselves.
