Obviously a good starting point to base lighting off would be the id maps. Although perhaps they are a bit too well lit in places. Also, with all the new options that modern light compilers provide, additional consideration is necessary.
I think in most of recent cases where a map looked overly or too dark for me, the reason was their use of colored lighting. Colored lighting is pretty much always darker than regular (white) lights, and it strongly depends on the color tone used. I find that maps that make heavy use of colored lights to light entire areas, quite often suffer from it as a consequence, especially if dark tones are used. More often than not the map will look fine if played without the .lit extension; although sometimes this can make them too bright even.
Fog is another factor that influences how a map looks, obviously. It can make dark maps more visible by highlighting the shapes of things in the distance, whereas without the fog the map would be very dark or even unplayable.
Both of these things are relevant since players use different source ports and those without support for .lit or fog may produce vastly different results.