I read the announcement and quickly looked at the B&H website comapring the D600 specs to those of the D300s and the D700. Though comparable to these cameras as to their dimensions, the D600 appears to be approximately half their weights. It makes me wonder as to how much of the D600's internals are plastic vs. metal.
I personally prefer a camera body with heft due to the size of my hands and because weight never really bothered me. I still intend to look at and handle a D600 at the camera store so this is just my initial observation.