The new DS Lite is smaller than the original DS, the stylus is 'fatter', there are more brightness settings, however GBA cartridges don't fit flush into the DS Lite, they stick out by 1cm.
The DS Lite screens are much clearer and brighter (even on the lowest setting) than the original DS. (this was the main reason for me buying one to replace my original DS) This Lite stylus is larger, both in length and thickness, The overall size of the console is smaller (and much more 'pocketable') And battery life is longer (but i cannot remember exact figures) I did take a couple of photos a while ago showing my brothers DS next to my Lite. I'll see if I can find them. EDIT: found 'em: Not the best pics ever, and my brother no longer has a DS (he also switched to a Lite) so i cannot take more