I noticed the smaller area when going from my DX D7000 to the FX D800. It is an issue for me when shooting portraits with the camera vertical. It is hard for me to fill the frame with my subject and focus on the eyes. Also, this is when shooting from my tripod. So no focus lock then recompose for me. It looks as though the D600 will have a bigger issue. People will need to get good at focus lock recomposing.
But like many things with the D600...You get to say...."it only cost me $2000".
My guess is that this will be an issue for 5% of the people on here 5% of their time. Nothing to worry about.