As a prior D4 owner (now into a D800e for my landscape work), I'm not impressed. It's a glorified firmware update masked as a new product launch. And even more surprising is the substantial cost increase.
Make no mistake, any incremental increase to AF system, speed and IQ capability is always a welcome addition (as is better battery, small raw feature, etc.), especially for sports, wildlife and PJ shooters - so it's not without some merit.
Frankly, I view it as a luke-warm interim release to buy Nikon Product Developers more time while trying to appease the pro market (but aren't all "s" versions sort of the same way). Had it been 24MP with better IQ (along with all the new D4s features like AF/speed enhancements) in a smaller footprint I probably would have been more enthralled.
It's certainly not a revolutionary change. And it's just skimming the surface of evolutionary IMHO. In this case, "s" stands for subtle, slight or small. But then again, every release doesn't have to be epic. Not much of a change for current D4 owners, but maybe worth it for D3s owners. My .02
As I posted on the Facebook announcement, while the D4S may be the new flagship from Nikon, it is an upgrade to the D4. If this were the next generation, it should be called the D5. Calling it the next generation is overstating what the camera really is!