The point of keeping voting donor only isn't a quality issue (though it helps) and it isn't meant to be an incentive for people to donate (though it might help), it's because I can trust the majority of donors not to try and cheat the system.
What you would see if I opened it up is users cheating their way to to the top and the whole system falling apart. With donations in the mix, it's harder for people to make multiple accounts just for the sake of voting their comics up. And if they do try that, they know their account could potentially get suspended.
Non-donors have nothing to lose by trying. And I don't have the time to sit around trying to catch that sort of behavior.
Comic commenting is a feature I'd like to allow non-donors to have eventually, once it's been tested with donors. But will come down to how much of a pain in the ass it will be to maintain.
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