I have owned the D300 and own the D7000. First the D7000 does have a Magnesium frame, and I have personally tested it on volcanic rocks (it passed!), and it is weather sealed.
Now I also own FX Nikons, so, I understand the need for the Bracketing, and low light abilities,
Now,, what had me sell the D300 and keep the D7000, it comes down ergonomics. First, I really miss the smaller size of the Old Nikon Film cameras. Pick up a F3 and marvel how it feels to the hand, compare that to my D300 and the D300 is like this heavy brick.
The D7000 brings back some of that small camera, high quality package. So, when, I am running around and want a lighter camera, its the D7000.
for the person that sold his D7000 for the D300 because of the his wife's images were s fine,, umm,, that is interesting. I do think that sensors generations have a look. I really find that my D700 and D300 really have a similar feel to the images. While my D7000 and D8000 have a different feel. Sometimes its almost subtle, and the way they handle the colors and range. I still love the D80, and my wife has that camera (and yes, I am often still amazed at the vibrancy and brialliance of its images). So, perhaps its the way we become accustomed to an outcome that steers us to a particular camera and sensor (kinda like the battle over film brands in the past).
In the end, they are all great choices, and really boils down to personal taste, and some hair splitting features. What is right for me, is not neccessarily going to be right for you.
My favorites - D7000 DX light weight run around with 1.5 X tele effect, D800 for when I am trying to get the most out of an image.
