Full disclaimer: I work for Vitalik and the Ethereum Research Team.
Here are a couple of reasons for why we are ultimately eschewing PoW:
(1) We are pushing to get blockchain updating down to 3 seconds. You really can't do this with PoW, since 3 seconds is our budget for our network overhead.
(2) A single block chain doesn't scale well - we're pushing for protocols that involve multiple blockchains processing transactions independently. The current proposals all rely essentially on PoS based on the central Ethereum Blockchain. Proof of Work isn't generally secure for these architectures - if a new blockchain was created then a botnet could easily gain 51% of the compute power for a while before hashing power could be directed from other parts of the network.
Vitalik says cryptocurrency developers should "reach out to LessWrong more", in order to not be ignorant of advanced computer science and game theory topics.
https://blog.ethereum.org/2014/12/31/silos/
I think Vitalik raises several major important points, namely:
* Bitcoin's $600 million wasted on electricity yearly in order to secure the network is a huge problem. Various Proof of Stake algorithms are attempting to improve upon this. The ideal solution would be one which combines the low cost of the Proof of Stake algorithm, with the high security or Proof of Work. I not enough of an expert to be able to say which solution is best, but both the Ripple consensus algorithm and the Bitshares Delegated Proof of State algorithm look promising. I am interested to see what algorithm Vitalik will choose for Ethereum.
* ASIC miners are a problem due to the resulting centralization. They have also lead to increased centralization in Bitcoin, as the network is now increasingly controlled by a few large mining pools.
* Blockchain technologies have a great potential to change the world, and solve governmental and organizational problems that society is facing. Beyond simply revolutionizing money, blockchain technologies could also be used in the future to support prediction markets, voting/consensus building, exchanges, etc.