And for the caves/ruins that do have light sources, I would like to see them stand out more. Create "patches" of dark, with well placed torches. Let enemies hide in the shadows. Let ME hide in the shadows. Let me remove torches with telekenises or a Thief inspired arrow, to move along the path undetected (other than a few NPC's made suspicious).
Create shady taverns and prisons with low lighting. This could also be introduced in castles. Have well lit throne rooms, but have corridors sneak in-able.
I also enjoyed the light effects that came from Morrowind's plantlife. And I believe the wilderness should be significantly darker. When I'm not in a town/fort/camp, I should struggle to see without a torch. I would like to be ambushed by bandits preying on the roads. Forget the chameleon effects of sneaking we've saw suggested, this, I believe, could vastly improve the abilities of NPC's sneaking.
I once installed "The Light Mod" for Morrowind. It has never left my mod list. Global lighting is reduced, light sources give off more light, I was given a Night Eye effect and greatly reduced sneak for carrying a light source and it improved my gameplay a lot.
I have always believed that lighting is the most important thing for visual quality. Without it, the game seems rather bland and uninteresting. Of course, players that dislike challenge always have the option to turn the brightness up and see everything always.