If not handled carefully, money can divide a project into in-group and out-group developers. If the unpaid volunteers get the feeling that design decisions or feature additions are simply available to the highest bidder, they’ll head off to a project that seems more like a meritocracy and less like unpaid labor for someone else’s benefit. They may never complain overtly on the mailing lists. Instead, there will simply be less and less noise from external sources, as the volunteers gradually stop trying to be taken seriously. The buzz of small-scale activity will continue, in the form of bug reports and occasional small fixes. But there won’t be any large code contributions or outside participation in design discussions. People sense what’s expected of them, and live up (or down) to those expectations.
— Karl Fogel, Producing Open Source Software