1. viewfinder. This is a big deal. Bigger, brighter. Much easier to take pictures. Much easier to manual focus and compose 2. 35mm lens is 35mm lens. That's a huge advantage for environmental portraits. On DX you really have to fight with the crop to shoot environmental portraits. There is just no DX way of shooting with 24/1.4, 28/1.8, 35/1.4 or 35/2 and achieving the same stunning depth of field images. 3. Much better high-iso and shadow noise performance 4. Deeper buffer. 7 RAW images on D7100 is a joke for actions shoots. 5. 24 megapixels and pixel density gives me huge cropping freedom for long-range shoots. This was the argument in D3/D700 days, but not now. I don't thing my birding pictures would be any better with a 24megapixel DX camera.
I love the AF coverage on the D7100 more. But it just can't compensate the other disadvantages it has. D600 is just much better camera than the D7100