Will, I'm not opposed to a plastic body per se. My N80 (that's an 'N' not a 'D') held up extremely well and I'm certain that if I put batteries in it it would function as new. I was and still am confident in its construction.
However, I like the feel of my D200 so much. Is it heavy? Yes. Large? Yes....especially with the L bracket. But the heft and the solidness is something that I like. And it has held up so very well through years of borderline abusive use. Once you are introduced to that sort of toughness you develop a level of confidence. I want that in my next camera. Are seals and a magnesium body necessary for such durability? I don't know. Is polycarbonate just as good? I don't know that either......but there must be a reason that the real pros seem to expect and demand it. That extra weight must have some advantage.