At any given time Google, through its various services and devices, knows, well, a lot:
What you’re searching for
What you are talking about when you email friends (and, through Google+, who those friends are)
Where you live, and work, how long you stay at either location each day
What you’re watching on your phone, tablet,or television
Where you are in the world, and, with Glass, what you’re looking at
And that’s just what I could come up with off the top of my head.
If you connect the dots here, it should be clear what Google doesn’t know, and where Nest fits in. Google has no idea what your energy consumption habits are, a data void that makes it impossible for it to make other, more significant insights.