So, this is the 5dMKIII competitor, not the D800. If the rumor about the price, $1500 is correct, then it is a 5DMKIII at half the price. Canon should be afraid.
Lol. Really? Max shutter a stop slower, limited use of magnesium alloy, 'very small and light', definitely an entry level FF camera, not a workhorse like the 5DIII. But who knows, since the D800 is a somewhat specialized camera vs. the 5DIII which is more of an all-purpose camera, the D600 may fill the same conceptual place in the Nikon lineup.
I see no reason for Canon to be afraid.
A lot of those features they've listed are expensive to produce, and the whole point of this camera is that it's supposed to be cheap. 100% viewfinders are complicated to produce. Weather sealing costs money. Full frame sensors are expensive. They're placing this in the price range of the D300? No, I just don't see it.
Nikon has been behind Canon in dSLR market share for years, and maybe they are feeling internal (or shareholder) pressure to change that - could be they feel the need for a loss leader. Amazon sells the Kindle Fire for less than the cost to produce it.