The new dedicated, right-handed INFO button brings the D300 up to the same level of usability as the D90. The dedicated INFO button lets you get to every camera setting and menu item using only your right hand! This button alone makes the D300s about ten times more valuable than the old D300.
The lack of this button is the main reason, as well as price, that I stopped recommending the old D300 a year ago when the D90 came out.
New Improved Rear Selector
The D300s uses the good rear selector, with the dedicated center button, from the D700 and D3, not the crappy old one-piece thing from the old D300.
New Dedicated Live View Button
If you use Live View (no one I know does), this is very important. The old D300 needed a bogus position on the rotating advance mode button on the top left of the old D300 to get in and out of this mode. Now, just tap the Lv button to get in and out of Live View, and while in Live View, tap the center selector button to start and stop movies.