I wonder if Google has an even greater (more so than with phones and tablets) inherent disadvantage to Apple because it's going to be beholden to a more generic UI given the diversity of devices by the android OEMs.
The Google card UI is slick and very user friendly, but it can stagnate pretty quickly given the potential for a more accommodating, dynamic UI that a company like Apple can make with a limited range of devices. I think it's not just a simple visual gimmick that Apple demonstrated a parallax effect as a major visual feature of iOS 7.0. If the iWatch is some sort of wrap-around your wrist screen, then layers of visual information could be really interesting.