I think an on/off switch would be handy. But then it might be the Freeduino, they have already done that. Anyway, a useful feature.
I second this suggestion. With auto-power select, it is too easy to disconnect an external supply and forget that power is still being fed over USB.
+1
Also I find there are times when I'd like to turn it off but unplugging and plugging in the USB get's annoying. The cable I use almost always twists so it's "upside-down". Very tiny pet peeve but one a simple switch would fix.
The perfect Arduino board has yet to be designed and built. However the Seeeduino boards are pretty darn close. 
Lefty