I hope you all realize what an extremely touchy subject you are on here. If not, do some research and come back. Check, for starters, the reports of various organisations around the world who have both volunteers and paid employees. Then check on the impact of rewards on productivity - hint: it's not what business monkey tell you it is.
There's quite a massive mental difference between getting a bit of thanks or getting specific pay for specific work.
This is very true. There have been some very interesting studies as well in late-fees vs. various forms of shaming.
I would think a bounty system would work best to defray these things. Players make donations (or pledge donations) to pr-approved projects. Devs can work on those projects if they want or not; once those projects are completed, the money is disbursed (maybe to the Devs, maybe not).
Properly structured incentives DO increase productivity. Eat-what-you-kill firms are generally more productive than lockstep firms, and bonus potential does have significant effects on worker output. However, you're right that poorly structured incentives (or monetary incentives that undermine existing social incentives) can have perverse effects.
Basically, I think there is a non-trivial amount of players who would like to donate non-trivial amounts of money (compared to current donations) if doing so could actually give them a sense of having a real say in game development, a sense of a stake in the game. This could have adverse effects, granted (entitlement can be bad). But it also has good effects.
Th mechanisms we've brainstormed might not be ideal. But the issue raised is: we have potential donors who aren't being tapped because our donation mechanism is inefficient. I mean, hell, I don't donate to my alma mater because they don't accepted tied donations of less than $5,000: small gifts thy require to be general purpose donations, and I don't like paying for the things they want to pay for. Same with my church: I donate generally out of obligation, but I'm way more generous and happy about my giving if it's to a specific purpose.
I guess that's what I'm getting at: having a specific purpose for donations does matter for donations.