The problem of full frame:
Vignetting at corners. When Canon 1Ds was first introduced, there are not many great wide angle lenses in the market for 1.5X crop DSLR, so 1Ds is a good add-on to utilize existing wide-angle lenses, but with the trade-off of vigetting at corners. See:
As of today, I am not sure how much sensor technology has improved in combating vignetting. Most new lenses are now specially design to give vertical light-path directly onto the sensor, giving the most of sharpness and contrast. I believe this will reduce vignetting at corners too.
When comparing D3 with D300, you might get same angle of view but depth of field will be different. A longer focal length will have more shallow DOF, while wider angle lens will have deeper DOF. So, you cannot get the same bokeh from D300 + 50mm lens with D3 + 85mm lens.
Another advantage of full frame when compare to smaller sensor size is that you can make better quality sensor DOT (pixel). A larger dot of each pixel is capable of capturing or receiving the light in a better way, hence give less noise and better contrast. This is why compact P&S camera with 10MP sensor is not comparable to DSLR of 10MP at high ISO. Most P&S Digicam will have bad noise at above ISO400. Just imagine a room will with balloon. A bigger room will be able to fit in BIGGER Balloon of the same quantity than a smaller room.
Again, vignetting remains a great challenge to full frame DSLR. Will need to wait and see how good D3 is.