I was about to say that someone on the photo forum posted about their D300 then I saw your test pics and you're the same guy haha! (I'm "DSLR_Noob" on that site) Congrats and enjoy the camera, if I see you at a meet, you have to show that thing off to me.
Also: I'll say what was said on the other forum: "That's a bad comparison." You have to get out the tripod, put the D200 on it, take a picture, replace with the D300 on the same tripod in the same place, swap the lens over, and shoot with the same shutter speed, ISO, and aperture. You can't change ANYTHING because in this instance, the baby might have thrown off the light meter. Also make sure they're both on manual, not one on auto and the other to duplicate with manual. Also put both in the same file size mode, if one's RAW and the other is large JPEG, converting the RAW with no post processing will make it look duller and flatter. Last thing, make sure the camera isn't on a parameter with the contrast or sharpness boosted. I only say all this because there is NO way the camera body alone makes that much of a difference as posted above.