I saw this thread kicking around the weeks but haven't looked or posted. I'm sure this has all been said, but anyway, my list in response to the original question posed...
1) Feature set; Unrivaled set of capabilities for the money.
2) Handling ; feels natural in the hands and so easy to use ... I will probably never buy another camera without dual wheels front and back.
3) Backward compatibility with decades worth of super sharp 'old' lenses.... if I ever want. Haven't yet, but great to know I can.
4) Numerous test image shots across many camera review sites, especially imaging-resource.com, held up really well against other cameras in its class or even the next step up.
5) Both SR and DR built into the body, to make lens purchases more economical in providing SR to each and any lens. I might have ended up with the D200 if it had DR (yes, D300 does now and I knew that would happen!!!), and if I could have stomached the cost of fast lenses with SR (VR in Nikonian).
6) Weather sealed body! And, at the time, the expecation of upcoming weather sealed lenses.
7) Fantastic value. The best value in DP today IMO!
...I bet if I tried I could'a made it a "top 10" list!