Another concern is centralisation, focused around China: "The block chain is controlled by Chinese miners, just two of whom control more than 50% of the hash power. At a recent conference over 95% of hashing power was controlled by a handful of guys sitting on a single stage."
This centralisation, he argues, betrays bitcoin's original. decentralised vision. "Bitcoin has no future whilst it's controlled by fewer than 10 people. And there's no solution in sight for this problem: nobody even has any suggestions. For a community that has always worried about the block chain being taken over by an oppressive government, it is a rich irony."