OK. Let's keep it simple.  These two cameras are identical in many ways. However, there are differences. One was designed with the pro or advanced amateur in mind. The other was designed more for the hobbyist or traveler.

The D200 has the following advantages over the D80. I will let you decide if they are important advantages for YOU.

1- True mirror lock up for better sharpness under some conditions.

2- Time lapse photography.

3- Larger buffer (19 raw verses 6 raw)

4- Magnesium body verses plastic.

5- Rubber body coating verses plastic coating. Better grip, easier to use.

6- 5 frames per second verses 3. Important for action, sports, and some wildlife photography.

7- More data in finder (Exposure, ISO).

8- A dedicated ISO button.

9- Takes CF card rather than SD. This makes it compatible with the D2x for pros.

10- More matrix metering segments (1005 verses 420) - which means potentially better exposures.

11- Shutter speed of 1/8000 verses 1/4000.

12- GPS input.

13- Wi-Fi option.

14- Shutter lag is 50ms verses 80ms. This would only be of interest to sports photographers or wildlife peak action.

15- Shooting banks and custom banks. Very important for the sometimes harried pro.

16- Bracketing 2-3 frames verses 2-9 frames.

17- Settable focus priorities.

18- Better weather sealing