Good darkness is all about ... great lightning. Because you just can't see darkness so it is suggested by the surrounding light.
I think Dead Space and Dead Space 2 may be the greatest looking games set in darkness. Don't drag me into the Xbox360 vs PS3 galore as to which one drives the greatest light and darkness... I already posted something about what still appears to me as a non-existent war (;-P)
The point is light comes to draw lines and suggest forms and shades of objects around. So... great darkness is all about lightning. It always seems to me that games which have no great lightning effects (it always appears evident in some more lighten areas) tend to have darkness which is too... let's say dark...
The way light reflects on the surface of objects or suggest shadows of the environnement or creatures is very important to me in the playability of a game. Let's say you are playing a Survival Horror game and there is this kind of creature that you never see coming. You will just be pissed.