One thing I never see discussed about the Nikon D700 vs, well everything else besides the D3, is the viewfinder! I just upgraded from the D300, and it's like a whole new world! I can SEE!!! Just like the F3,F4 and F5 the D700 eyepiece is so much better! That alone was worth the cost of upgrading. If you wear glasses it's a necessity!
...and the eyepiece shutter for those of us who do long exposures is a huge advantage as well! I used to use a D300 as my primary shooter as well, so I know where you're coming from. It's a whole new experience!
A quick suggestion: get a Hoodman eyecup. It eliminates light as you're looking through the viewfinder. Makes a huge difference in bright sunlight. You can pick one up for about $20.
I do not understand your comment - unless you have bought a fast new FX lens and you are comparing apples to oranges. I use the D3s and D300s alongside each other several times a week. The D3 viewfinder is only about 17% apparently larger than the D300 - reduced from apparently 50% larger because of the different minus viewfinder magnifications. The D300s viewfinder is brighter by about 1 stop. I am not disputing the D700 extra 17% apparent image size (ignoring the viewfinder edge crop) is not useful, but the D3 and D700 have a dimmer viewfinders than the D300. Overall I prefer the D300 viewfinder.
Photography is a bit like archery. A technically better camera, lens or arrow may not hit the target as often as it could if the photographer or archer does not practice enough.
The D300 eyepiece has a higher eyepoint than the D700 - 19.5mm as against 18mm.
With my spectacles resting against the standard eyepiece surround, I can see the whole frame (including the display across the bottom) on my D700 and my D300. Using the two cameras alongside each other, I can see very little difference in viewfinder brighness, but if anything the D300 looks brighter to my eyes.