Chris, I think your question is an excellent one. The D700 is nearly twice the cost of the D300 with the same body and nothing really new combined to the D3/D300 options already available. Even so, there are a few things that make me want the D700 over the D300 (and even a D3) when I get a new body later this year.
1. D700 pop-up flash. The lack of an on-board commander for the CLS flash system in the D3 is an issue for me. I find the pop-up flash of my D200 so handy at times. My D2H lacks this quite useful feature. More importantly, when it is used as a commander for my SB-800s the D200 flash is marvelous. This built-in capability is noticeably lacking with my D2H "Pro" body. The D200 flash is always available, easy to enable, and works without adding weight to the top of the camera. With all previous Pro bodies you had to have a SB-800 or SU-800 attached to use the CLS multiple speedlight system. Like the D200/D300, with a pop-up flash on the D700, that issue is resolved. Minor issue to some people, but an important one to me.
* OK, so the D300 has the same flash, why not stay with the cheaper body? Because from what I have read, the image files from D3 sensor are generally better at all levels. The difference in very minimal to significant at low ISO depending on what you read, but it is not contested that as ISO goes higher, the D3 sensor really shines. Which brings me to #2.
2. IQ If not THE best, the D3 has one of the best DSLR sensors and supporting chipset units made to date by any camera company. There are individuals who have written about going from D3 to D300 and back to D3. Why? Because the D300 while a superbly built and lighter camera, plus a fantastic bargain for the price, just doesn't match the D3 image quality. It gives the D3 a good run for the money and is much lighter and easier to tote, but it isn't a D3. The D700 has the same image sensor as the industry leading D3.
3. $2000 - Cash or credit. For roughly $2000 less than a D3, you get all of the advantages of the D3 sensor and all of the advantages of the smaller, lighter, D300 body. Including the optional MB-D10 battery pack when needed. Go light or go heavy as your needs or desires dictate. It is the Olympus OM-1 of our time.
4. The viewfinder of the D700, while displaying only 95% compared to 100% for the D300, is still bigger and potentially brighter. The eyepoint location and pro body setup of the D700 viewfinder appeal to me more than that of the D300. The D300 viewfinder is pretty darn good to begin with, but one gets spoiled by a pro body viewfinder and the D700 has one.
5. FX and DX capability. Wide-angle lenses are as wide on the D700 as on the D3 and you have the option to use DX lenses in crop mode as well. Not available on the D300.
For me the D3 was almost perfect, but not quite there. It is really big, pretty heavy, and rather expensive. In addition, the lack of built-in capability to control CLS speedlights was a functionality that I felt was sorely needed in the Pro body line. Not a pop-up flash necessarily, but something to take advantage of the SB-800/SB-600 CLS capabilities in the same way a D200/D300 can. The D700 has that. And the D3 sensor. And FX. And DX. And the lighter packaging and size of the D300 body frame.
For $5000, you can have the major IQ advantage of a real D3 sensor and an optional 24-70 f2.8 for the price of a D3 alone. Nice.
"... you see, but you do not observe." - Sherlock Holmes