There's a very small set of problems which are difficult to solve but easily verifiable. This is necessary to prevent cheating. I think there would be much more cheating in the distributed computing projects if there were $$$ at stake. However, I think your idea is on the right track. There are plenty of distributed computing projects out there which are more practically useful than, say, SETI. If there is a way to verify proof of work for something like protein folding in a decentralized way, I'm sure there's a niche for a "socially conscious" crypto-currency.