I don't have any screenshots, but here's one thing I've noticed at the lowest setting: at dawn or dusk, you no longer get unnoticeable transition from light to darkness; it gets dark in only a few noticeable increments.
When in the Swamp doing the Monoliths Quest I noticed the light didn't rise to the sky from the obelisks. I wondered (wtf) was playing on low light due to the BLURRY issue, so changed it back to med and (wonders) the light rose to the sky once again. What a give and take predicament. Please find a fix for the blurry after cutscene issue.
I have been comparing high and med recently, and it seems that there is really small difference on light sources the ambient light is more pronounced. I noticed that in the jail in Vizima on fireplace or torches on the walls. It is not that the ambient light is missing on one and present on another, but just seems more "dynamic" and "alive". Do not know how to describe better 
. Apart from that I cannot tell difference either. I am playing on medium due to pixelation bug (on HD4870 ).
. Apart from that I cannot tell difference either. I am playing on medium due to pixelation bug (on HD4870 ).Of what I've seen, the difference between full and med is mostly that full is "warmer". Between med and low the difference is much bigger though. I took a few shots in the beginning of the game when I tried to get rid of the blur problem (another thread), they are not exactly the best shots demonstrating light, but compare the window, the hole on the wall and the lamp to the left and you'll get some idea. Lighting on lowest setting makes the game look quite dull.FULL
MED
LOW
MED
LOW