I really do think that I get better pictures with my D700 than I did with my D300, although to be fair to the DX body I very much appreciated the 100% viewfinder and better AF coverage.
How are my pictures better? On some images taken during the day with my D300 I've noticed that with very bright colors, especially when lit along the rim, the tones seem to merge together in a way that I find difficult to describe. They seem rather cartonish--very saturated, with the transitions in tones or edges of objects smeared together. I've never seen this in one of my D700 shots, and I shoot this type of scene frequently enough that it should have happened by now if there isn't really a difference in the cameras' respective capabilities.
I've also noticed that D700 images with blown highlights (doh!) are so much easier to turn into something usable.
Lastly, the D700 meter seems to deal with backlighting a little better--meaning that I get fewer underexposed backlit subjects.
It's such an awesome picture making machine that I'm totally convinced of one thing: if a picture isn't good, the problem isn't the camera.