D700 a great camera, but now becoming an old design. I always was not happy that they did not give 100% in the viewfinder, but really hard to find any fault with what is now a classic camera that will slowly slowly fade from the scene. I use mine as a rugged outdoors camera.
but,, the D800 is fabulous, I have been totally pleased, after owning 6 Nikon DSLR models, I am convinced, that it will be another 5 years before another model comes around to drive me to another body.
By the way, the D700 was the primary camera for around 4 years, so the model refresh is lengthening.
it always tempting to say,, well this is all that one needs/wants. but who is to say what will happen 5 years from now, as the sensor technology continues to improve, and features improved. I predict a HDR control, where one dials in the dynamic range with the SINGLE shot. but,,, it might be splitting hairs.
In 5 years, processing speeds will be even faster, so, could see 10 fps,,, 50 megapixel, 3D processing, ultra fast mirrorless focusing via live view, but none of these features will make you a better photographer,, that you can't get with an used D80, and a beaten up lens.