The viewfinder is huge, and is so much easier on the eyes! If I shot with the D300 all day my eyes would get sore, but the D700 has proven to be better so far! Aside from the absolutely outstanding high ISO noise performance, one of the big pluses of the D700 over the D300 (non "s" model) is the virtual horizon in the viewfinder, which I have set to come on when I press the FN button. One thing that I found interesting is that the screw driven AF motor on the D700 seems to be much quieter than the one on the D300, not to mention that AF is snappier in general.
As for the 50mm F1.8D, I've had it for a almost three years now so I'm somewhat familiar with its performance. Of course FX is a new ball game, and the lens acts differently on the D700 than on my film camera. It is noticeably soft wide open, and really needs to be stopped down to F4 to be useful on an FX body, vs. F2.8 on a DX body. I'll play around with AF fine tune, and if that doesn't help, I think I might sell it and get the 50mm F1.4G.
After more testing I found that the 35mm F1.8G is great, as long as you are shooting at close focusing distances, but once you start shooting subjects that are more than a couple of meters away the vignetting is just too much. It is tack sharp, even wide open, much more so than the 50mm F1.8D, by a landslide.