I'm curious how the "always listening" will fly. This is another example of technology that is far ahead of the sociological issues which will take a while to resolve.
(On that note, I feel like every day our newest technologies stretch farther and farther away from regular people, and it will become a major issue of our generation.)
I am personally most excited by quick access to the camera. I'm curious what 10 megapixels will look like, but typically this is total BS, and a waste of pixels, you can usually reduce phone photos to 70% of their original size, which means throwing away 1/2 the pixels (and file size).
Now, my biggest issue: upgrade my terribly creaking Nexus S to a Nexus 4 right now, or wait a month or so for this swanky new Moto X?