Well, this is a valid question, and one I have heard even some pro's ask. In fact a number of my friends professional photographer friends have opted for the D300, seeing it as better value for money.

I will list a few of the major differences here:

The D3 is a full frame camera, meaning that a lens of a given focal length, say 200mm will perform at 200mm. The D300 uses a smaller sensor size meaning that the D300 magnifies the lens to about 300mm.

While this is good for sports photographers and wildlife photographers who use long lenses, it is less than desirable for those who want ultra-wide view-areas.

The larger sensor on the D3 also mean that it delivers less depth of field for a given f-stop and that the camera generates less noise in low light.

Then there is the fact that the D3 has a built-in vertical grip. Though the D300 has an optional grip, this is not as well balanced.

The D3, in cropped mode, can take up to eleven frames a second, it loads two compact flash cards rather than one, and the shutter is made of more durable material.

In all, the D3 is a workhorse, and even though it does not appear so on paper, you get a lot more for your money, the only decision is whether YOU will gain from its added features.