Ethernet: many pro attach the body straight to the laptop.Right now you can do this through USB, but D4 allows you to attach straight to a network drive. For example, if you take photos of 100 meter hurdles at 11 frame /sec, you can download 100 frames directly to a network drive, archive and distribute them to the headquarter in another city.
Uncompressed video output: it is what movie industry want. You can figure out the reason.
Face detection: my guess is that it is for photojournalism and sports.
Keep in mind London Olympus is 6 months away.