I realize this by itself is a ridiculous statement. I could make the same list as you, covering the technical advantages of the D700 over the D90. There is no doubt in my mind that I can find 100's of photos here and on other photo sharing sites, taken with the D700 that blow away anything I've ever done with my D90. So let me add a little perspective to the title. I rented the D700 for ONE DAY. Why my D90 is better FOR ME RIGHT NOW.
I've had my D90 for about 2 years. I know the camera well. I know, without looking where buttons are. I know what settings I change for different situations and can probably make those changes without looking at my camera if I had to. Looking at a scene or subject, I can visualize the zoom range of each one of my lens as if I were looking through the view finder. I know the technical limitations of my D90. I can see in my head the amount of noise I'll introduce with each increase in ISO. There are no surprises left for me with my D90.
I rented the D700 because I wanted to see just how much better it would handle higher ISO's. I was shooting an event inside of a gymnasium where flash was prohibited. Ugh! The D700 didn't disappoint. There is a remarkable difference in noise between the two bodies in the 3200 - 6400 ISO range. That however is not the point. So on to it.
I shot somewhere around 1500 images in a 14 hour period. I spent some part of that time changing settings. I spent some more of that time looking for buttons and menu items to make the changes. I spent considerably more time reviewing images on the LCD than I would shooting with my D90. I spent too much time thinking about the camera and not about making images. I had this beautiful D700 in my hands. I would dare say my dream camera, and I was uncomfortable with it. In the end, I made some decent images but I never really got comfortable shooting with it. I had my D90 with me and took it out a couple of times. The noise was noticablely higher but I was comfortable with it in my hands. I'm sure given enough time, I would eventually have the same level of familiarity with the D700 as I do with my D90. It hit me though, other than the increase in noise at higher ISO's there really are no limitations for me with the D90. The fact that I have such an intimate relationship (insert snikcer here) with my D90 makes it the best DSLR body there is....for me. Having such a firm grasp on exactly what your camera can and can't do and how to work within whatever perceived limitations it has, allows you to concentrate completely on making images and not the camera. I will put that dream of having a D700 to bed for a while.