>The problems that crypto-currencies attempt to solve are more to do with monetary policy. E.g. - you can't have things like quantitative easing / manufactured inflation if your money supply is dictated by a tightly controlled algorithm.

If that's what you want... the extralegal aspect of cryptocurrencies is a big problem. I still have a few hundred bucks tied up in e-gold, which was perfectly setup to do what you want, and backed by real gold, but as a side feature was also convenient for black market transactions.