Found a great article that explains why proof of stake isn't a consensus mechanism, but a cunning way for an electronic cash system to operate: Proof of stake is a scam and the people promoting it are scammers.

I hadn't thought of this “Nothing at Stake problem” he speaks of. In proof of work, there's a punishment to those who misbehave, but not in proof of stake. @TangentC, wanna give this a read and tell us where he's wrong?