I've been using my new white ds lite for a while now, and I can say that the sound has been improved. It is louder with slightly more clarity. The diagonal is actually useable. In games such as Golden Sun, I don't find my character running diagonally, then slipping back into running in a straight line.
The screen backlighting is more uniform. Previously, the colours at the top of the touchscreen, looked slightly more washed out, and you could see the light emitters in some of the corners of the screen. That has been fixed.
Finally, the abxy buttons are less mushy, and have more feedback.