What are the best criticisms of blockchain technology?

What are the best criticisms of blockchain technology? Specifically, what are the top flaws of the blockchain as it's currently implemented by Bitcoin?

A few I can think of:

double SHA-256 proof-of-work isn't computationally useful for anything (GridCoin is still affected by this criticism to a certain extent);
it's not as immune to centralization as it could be (control of blockchain is proportional to computing power; it should be proportional to number of human users).