What if I gave you $150,000? Would you make the product to my specifications? Does this illustrate why this could be a channel to disrupt the spirit of open source software?
Imagine Github does implement this. Now you have a bunch of coders who want to work on "open source" projects for money, hoping to get backers as if they did a Kickstarter project. Or maybe it could start an OSS Github "gold rush."
Now that we have a bunch of money-hungry developers, it'd be pretty easy to de-rail or commandeer projects simply by donating a large portion of money. Now imagine a non OSS competitor starts contributing. Multiple things can happen:
1) Any innovation or development of features could be easily copied and incorporated into competitive projects for sale.
2) A developer gets "hooked" on that donation money and starts favoring the large donors' requests over others.
3) These requests could sabotage the quality or the focus of the project.
4) The donor could pull funding at this point and collapse development and support of the project.
Full blown conspiracies aside, the whole point of OSS was to open up development for everyone to grow and learn, and to remove software development from the corruptible influence of money.
I think others on this thread had a great idea, make it a "tip", and make it anonymous. We would still have to limit "tips" because anyone can make a huge tip and then e-mail a "ransom" letter to the developers to do stuff if they want to continue getting large tips. The previous ordered list applies here.
Again, I'm going to an extreme and likely this sort of abuse wouldn't be rampant in a system like that. But involving money at all directly into the process sounds like it could do more harm than good.
If you want to take donations, you can do a Kickstarter to get up and running and then take some sort of "beer" or micropayment service on your website. Github integrating this into their offering seems irresponsible to the OSS community at large.