On D200, check for banding. I have a young friend that bought an early one without saying anything to me and the first thing we did was shoot some high contrast scenes and there it was - major bad banding.
His also had a problem with the battery door not latching positively and the battery popped out the first time I slipped the camera into my tripod. Turns out the door spring was broke - ordered a new door and that was an easy fix.
Make sure bracketing works, WB adjust properly.
Another is CF slot issues, I've seen that pretty often in D70/s model cameras. My own has an issue with any card I have - though a reseat usually clears up the problem. I have heard the camera can be disassembled and the cable reseated on the card slot.