It turns out that the combination of empowering fans and offering desirable features is incentive enough for people to donate substantial amounts of money for something they could receive for cheap (or acquire elsewhere for free). The special sauce? Making the whole thing easy to use. Humble Bundle aimed to make it painless for a user to pay what they want, allocate their payment, and receive their games. (They even made it easy for users to increase their donation amount after downloading the games!)
Also, Atom Zombie Smasher has been updated with bugfixes.