Why would I want to contribute to someone's project that I don't even use? What would inspire me to all of the sudden want to hack on git's internals and then not dive into the source? On one hand, yeah, finding some menial bug or documentation fix I can work on to start contributing to a project is great, but at the higher levels just presenting tasks from issue trackers for me to smash, I don't think is a great idea.
Look at the ActiveRecord/AREL rewrite, sure @tenderlove was just squashing bugs in the Lighthouse tracker but only because, as he says, "AT&T Interactive started paying me to work on Rails" (http://engineering.attinteractive.com/2010/10/arel-two-point...) If you don't dive in real deep you can't really make things better and that takes a lot of time. You end up investing a lot in the project and the people around the project. So I don't see where GitHub doesn't support this. How does your product makes any of this any easier/better/more full of awesome?