What is the novelty in AlphaGo, Google Deepmind's Go playing system?
The existing programs before AlphaGo were based on:

1) Convolutional neural networks (CNN), trained on a database of existing games.

OR 

2) Monte Carlo tree search (MCTS)



AlphaGo is based on a combination of:

A) reinforcement learning: train networks by letting versions of CNN's (see above) play against eachother.

AND

B) MCTS using moves generated by step A)

On top of that, the performance was improved even further by using distributed computing with large amounts of CPU's and GPU's.

So the novelty was the combination of the above techniques A) and B).