One extremely convincing data point for starting a stampede of contribution would be, "Person X made $Y/hour for their contributions to projects on assembly"—even if it's pointing to one amazingly valuable contributor, one exceedingly successful project, and one uncharacteristically highly-valued bounty.

Right now it's a challenging mental math leap of faith to make the decision to spend _working_ time on projects there, but for _learning_ / extra time it's already a no-brainer (that's why I've contributed at least).

Of course I imagine this is all stuff that's already being thought about and is more of a later-thing / difficult to bootstrap, but worth keeping a focus on. Top $/hr made by a contributor on the site might be a solid metric to optimize for.