I love the visual direction, and I think it will work very well for a diverse set of Google products. The style guide is beautifully done and well articulated.

In terms of interaction design, however, it doesn't break any new ground, apart from better transitions (but at times, too much).

> So the team set about creating this metaphorical material, which is a little bit like magical paper.

A bit of nitpicking: "a bit like magical paper" can hardly be called a Metaphor. It's just a loose analogy. In fact, I have to force my imagination to see the paper-ness in the UI, otherwise it just feels like flat UIs with realistic shadows.