>Unless someone is into very wide angle photography, below 20 mm focal length, or >mainly shoots under very low light conditions, what is the advantage over a D300?
Hmmm...let's see...say you are shooting an event in the shade on a partly cloudy afternoon- the light is only slightly lowish. Thing is, you can't get too close to your subject and you are shooting with your 24-70 all the way zoomed-in. The shutter speed you need to stop the action is 1/250, and you need good depth of field, say f4.5-f/5.6. On the D300, you can do it, but you must sacrifice some DOF, accept a higher noise level, or use flash. With the D700, you get super-clean shots, high shutter speed and good DOF.
On the other hand, if you shoot birds, you lose 1/3 of your focal length AND you lose sharpness outside the center of the frame due to the loss of the 1.6 crop. TC's don't work nearly as well for the same reason. You can crop, but then you are left with a 5 MP image where a D300 would have given you 12. Full frame cameras are a poor choice for long telephoto shooters.
Different tools, different uses. One is not better than the other.