"Look and feel" of areas seems to be one topic here.
"Actual IG effects" of the level of illumination in an area is another.
After understanding both of those, you can start to throw a wrench into things with all the problems like "what about time compression" where planning in a tavern costs you 18 hours, or "why does NWN1/2 stealth work in the middle of a noon-time road or in the darkest of dark areas exactly the same, or work the same against a blind human as against a dwarf with Ultravision cast on him?" and all the other situations you can think up.
Anyhow on look and feel, that's builder license / discretion, if they want to explain that 95% darkness in an area is to reflect long sizzled out light spells, phosphorescent moss, whatever... all dandy. Don't care. For now, it purely creates convenience / inconvenience for actual PLAYERS who have a devil of a time navigating an area without contrast/gamma/etc. tweaked on their graphics options. No effect on the PLAYER CHARACTERS, except any voluntary RP effect, like "Hoo boy it's dark as a witch's heart in here!" vs. "Ach, I'se threaded a needle in mines blacker'n thissun since ye were 'pon yer mudder's teat!"
The "numbers side" is the other part. NWN games don't do squat about illumination level UNLESS you're caught in an actual Darkness spell, without a magical countermeasure (and the Darkvision or Low-light Vision racial feats don't cut through it). So basically two questions get asked:
(1) When are you in a tough environment?
(2) What are the consequences when in a tough environment?
The first question is mostly determined by light sources related tables, though the numbers presented on them sometimes get multiplied if you have relevant light related feats. Basically you'll have a source, a small radius usually of "bright" illumination, a larger radius of "shadowy" illumination, and beyond that, you're dealing with full-on darkness. See the table here (
http://www.d20srd.org/srd/exploration.htm), unfortunately while it covers torches, lanterns, various spells etc., I can't find where (if anywhere) topics like starlight, moonlight, dawn/dusk/twilight might get covered. It may simply be assumed that outdoors during the day is always in the bright category to pretty much infinite range, but night I'm baffled on, there's probably a rule on how little shadowy illumination you can expect in a forest under the moon.
The second questions are answered in summary at the same link above.
Everyone's operating at their best in bright light, but HIDING is impossible absent cover/invisibility.
In shadowy illumination, not much changes, except that critters within such area of shadowy illumination get concealment RELATIVE TO THAT OBSERVER (which means miss%'s as well as the ability to HIDE on someone). All I mean by that relativity statement is if a human, travelling with an elf and a dwarf had a hooded lantern in an otherwise pitch dark cave, the human's dealing with 30' bright, 60' shadowy, the elf's low-light vision doubles such ranges for 60' bright, 120' shadowy, and the dwarf's darkvision simply means that the first 60' are "as good as being lit" for the dwarf, even if the area was fully dark. So really this means he doesn't care about the 30' brightness of the lantern, it's still bright for him. The extra shadowy illumination out to 60', well, for him, it may as well be bright as well. However, at the 60 foot mark, it cuts off to darkness. Even if the lantern was extinguished, the dwarf would still have 60' brightness, then after that, full darkness, basically ignoring all the shadowy stuff. Yes, this means the elf is technically seeing farther than the dwarf, which may sound like sacrilege, but this is the difference between low light vision and darkvision... one means you get more mileage out of a lightsource, one means you can deal without one completely.
Anyhow, off of shadowy onto darkness. I'll cut and paste since it's number-heavy:
"In areas of darkness, creatures without darkvision are effectively blinded. In addition to the obvious effects, a blinded creature has a 50% miss chance in combat (all opponents have total concealment), loses any Dexterity bonus to AC, takes a -2 penalty to AC, moves at half speed, and takes a -4 penalty on Search checks and most Strength and Dexterity-based skill checks. "
I'll mention just due to the words "total concealment" being there, that if you're within an area of darkness relative to that observer, you can hide on them. Again with the mixed race party with a lantern above, if some kobold is hiding 100 feet dead ahead of that party, the human can't see him no matter what happens (over 60 feet means full dark), from the elf's perspective, we're talking within 120' so it's a shadowy area, yes the kobold is allowed to hide, and the dwarf beyond 60' is looking at plain darkness as well. Elf may be able to make opposed Spot checks vs. Hide at least, and if they approach, eventually the kobold will come within less than 60 feet, which instantly means the Elf (who's got "bright" vision to 60' due to lantern and LL vision feat) and the Dwarf (who's got darkvision) will see the kobold, absent any cover (like hiding behind a rock) or invisibility. Human would auto-spot at 30 feet, again, absent cover or invisibility.
Note this is also why you do not HIDE RIGHT IN FRONT OF SOMEONE. You sneak up on them from behind, back flanks, etc., while they're not looking at you. It's also why you don't sneak through areas without cover in daylight, pick a warehouse full of crates (aka cover) as opposed to an empty warehouse if needing to sneak up on people, why capers happen at night not in the day, why you'd steer very clear of elven sentries as opposed to drunken human night watchmen, etc.
Anyhow, back to full darkness. A dwarf of half-orc is basically NEVER going to get blinded by non-magical darkness. Stick them in a completely dark room, they can fight up a storm, search for secret ways out of such a cruel trap of a room, etc. You never exactly fight 60+ feet away from yourself in melee, right, you'd need long arms. Still, I guess you could shoot a crossbow into the absolute dark and deserve all the blindness penalties (concealment miss chance against whatever AC the opponent you believe you "heard" out there has). Same with if someone shoots their crossbow out of a dark area at you, your AC might be penalized, rule doesn't seem very clear to me, but it'd make sense to say if you don't know where an attack could come from and when, it's hard to react (hence Dex bonus lost and AC gets -2 generally).
Everyone else though could end up in the dark fairly often, basically as soon as they lose a light source (invest in glowing weapons people, you'll always then be bright/shadowy for a reasonable melee distance). Wham, all kinds of penalties, though things like a blind fighting feat ease the sting in a big way (
http://realmshelps.dandello.net/cgi-bin ... -fight,all)
Anyhow, a pretty long summary of the PnP side of things, checking out those links though and all the cross references, and figuring out "how does a mixed party operate like this?" makes for a healthy dose of reading.
If people want to go into the 1000 problems of implementing much of this into NWN, have at it. NWN lacks cover completely and concealment almost completely, except as may be applied by spell. "Stealth mode" ignores every element of the environment, as you see from the PnP stuff, it's impossible to be stealthy under someone's brightly lit nose, absent cover (like being behind a crate), so the many rants we've seen over the years of stealthing right into someone's face for a CvC are pretty darn justified. "Tavern" stealth is a virtual impossibility as well unless the tavern's lit by maybe two or three shadowy illumination-providing candles, as soon as you have a torch, you've probably brightly lit virtually the whole common area, though booths etc. might start to provide cover. NWN also lacks any "blind without light" mechanics at present, I can imagine people trying to make compromise solutions perhaps for the indoors, and maybe a weaker system outside at night "just in case" the moon is full or there's cloud cover vs. starlight etc., maybe something would be workable to basically penalize most races in the dark as if full blind, unless they can at least produce a candle, in which case, they're safe. Dwarves and half-orcs can start stealing people's matches to feel superior
By the same token though, human PLAYERS are still probably going to be able to navigate their blind PC very safely to doors and stairways out of a black pit, because a game where everything's blacked out is probably fairly unfun. Their PCs I guess will just be "really lucky" about eventually bumbling their way out of bad situations, though if they get into battles in the dark, possibly with blind mobs (hey, even Dire Rats only have "lowlight" and "scent", neither of which cuts the blindness penalties for full darkness), so maybe everyone will get a few boo boos.
Anyhow, damn long summary, but I can't begin to imagine how much scripting could be involved with different categories of "brightness and shadowy illumination" casting relative to all different races, how you'd do indoor vs. outdoor, etc. Maybe a flat "absolute darkness indoors unless have a light source" thing could be figured though, some script to check if indoors, if night, and ignore arguments that'd be bound to come like "this castle we're raiding at midnight has WINDOWS which should let in a bit of moonlight, we're getting our asses kicked since we forgot a torch"... ah well. Tougher topic than weather I bet, since lights can be turned out anywhere, not just tied to glaciers, deserts, volcano, whatever.