In uniform, shared light levels, vision penalties due to darkness work fine, intuitively, and exactly as written (no surprises). The deeper the darkness, the more trouble you have seeing, hitting things, and protecting yourself.
This, however, isn't the case you're asking about.
Mixed lighting conditions
Let's stick with RAW to analyse this situation of two people in different lighting conditions. The results are almost entirely the same whether whether Aspartame is in magical darkness or the (possibly very dark) shadow of an oak tree. The results are going to be weird.
These results are pretty counter-intuitive. In real life, the rogue sneaking through the shadows has an advantage on everyone else, and you've probably handled it as such — the rogue themselves isn't blinded by being in the shadows, but they are in Pathfinder!
Pathfinder's vision penalties are counter-intuitive and weird because they don't correctly model vision as we know it: they give you trouble depending on the light level you're standing in, instead of giving you trouble based on the light level of the thing you're looking at. In reality I don't have trouble reading things in the dark because I'm in the dark, I have trouble because those things are.
Like I said though, they work fine when people are sharing the same light levels. They just weren't written with differing lighting conditions in mind and don't handle them well. The sacred RAW should be burned in this case upon the altar of This Could Have Been Written Better For These Cases. Rules are worth the results they produce, and here the results they produce are kinda dumb. Those who wish to apply it exactly because those are the rules can go ahead and suffer the headaches.
Drawing sense out of the ashes.
Pathfinder should probably just be modelling how things really work: darkness affects you based on what you're looking at, not what light level you're in. Accordingly, I'm making a recommendation that in mixed light levels, the RAW be interpreted for what it was probably intended to do — which just requires a small adjustment of the original RAW:
- Dim light: Being in dim light doesn't make you have trouble seeing, but looking at things that are in dim light means you have trouble seeing those things. Past that though the RAW works fine: things in dim light have concealment and you have a 20% miss chance against them.
- Darkness: Being in darkness doesn't make you blind or suffer penalties. However, you are considered blinded for the purposes of observing or interacting with anything that is in the dark. You have a 50% miss chance against those things in combat, you have penalties to AC and perception versus things in darkness, and so on.
- Alternately, stick with the Darkness spell's function, and just grant things in darkness total concealment and forget about blindness.
Wyrmwood brings up in comments that due to firing through concealment, firing past/through dim light introduces a 20% miss chance. This causes sense-making problems if you're on a long archery range, and a tall tree off to the side is casting a shadow across part of the middle of the range: those rules assert your target dummy now has concealment, despite still being in the bright daylight. These rules work fine for actual obstructions like black smoke, but not for light. I suggest that for the purposes of firing through concealment, don't count dim or dark squares — just count what it is you're targeting.
That produces these results, which are probably actually intuitive:
- While Aspartame is in dim light:
- Aspartame is considered as having concealment from Xylitol. Xylitol has trouble seeing him, and has a 20% miss chance.
- Aspartame fires without difficulty, because Xylitol is in bright daylight and clearly visible.
- When Aspartame is in darkness:
- Xylitol is considered as being blinded for the purposes of trying to observe Aspartame. He has a 50% miss chance trying to attack Aspartame.
- Aspartame still fires without difficulty, because Xylitol is in bright daylight and clearly visible.
- Xylitol has all the defensive and sensory penalties for the purposes of any interaction between himself and Aspartame, because he can't see what Aspartame is doing at all.
We get these results, again, whether the dim light or darkness is due to magic or the shade of an oak tree.
Breaking your overall question down you ask:
Would someone with darkvision or Devil's Sight be able to discern those lighting differences we consider important to our normal vision?
Would someone with darkvision or Devil's Sight be able to detect where subtle shadows are when there's a faint light source to cast them?
Do the rules have anything to say, or is there guidance from closely connected material or other editions available?
Darkvision
A discussion of the difference between these the rules describing darkvision can be found here what is the correct interpretation of darkvision and the rest of this answer will assume that darkvision is consistently applied as written in the race descriptions (e.g. dwarf PHB p.20) though it actually does not make much difference to the answers.
Darkvision PHB p.20:
You can see in dim light within 60 feet of you as if it were bright light, and in darkness as if it were dim light. You can’t discern color in darkness, only shades of gray.
There are two specific levels of light covered here:
- Dim light, where there is a source of light
- Darkness, where there is no source of light
In both cases darkvision enhances vision such that "you see as if it were [the next higher level of lighting condition]". Obviously you can also see at all light levels in between using the same "as if" rule and there can be no shadows if there is no light source. As always the light level present are adjudged by the DM.
Addressing your questions one by one:
1. Would someone with darkvision be able to discern those lighting differences we consider important to our normal vision?
Using the "as if" rule YES, with the exception that colour is replaced with shades of grey in darkness. If a creature with normal vision would be able to discern the lighting difference in the "as if" lighting condition, then so can a creature with darkvision in the actual lighting condition. Otherwise they can't.
2. Would someone with darkvision be able to detect where subtle shadows are when there's a faint light source to cast them?
The question appears to be asking "would you be able to see subtle shadows if the lighting conditions were less than dim but more than darkness?"
If the DM adjudges that a creature with normal vision in the "as if" light level would be able to see the subtle shadows in question then YES, a creature with darkvision can see them in the actual lighting conditions. There is obviously no sudden jump between darkvision's benefits in dim light to those in darkness, it is continuous.
Devil's Sight
There is a discussion about how the Warlock ability Devil's Sight works here.
Warlock's Devil's Sight (PHB p108):
You can see normally in darkness, both magical and nonmagical, to a distance of 120 feet.
This lets you see in total darkness as if it were full light. You gain no bonuses in dim light from Devil's Sight so the answer for darkvision above applies. However for full darkness you get "normal" sight. So it is full colour and any enhancements (goggles of minute seeing) also work. However as there is no light source there will be no shadows. Weird.
Addressing your questions one by one:
1. Would someone with Warlock's Devil's Sight be able to discern those lighting differences we consider important to our normal vision?
In dim light no. In total darkness yes, with the caveat that there would be no shadows. Darkvision is a separate ability.
2. Would someone with Warlock's Devil's Sight be able to detect where subtle shadows are when there's a faint light source to cast them?
No, it has no effect in dim light, only in darkness. Darkvision is a separate ability.
A devil's Devil's Sight works differently, however. For instance the Barbed Devil (MM p.70):
Magical darkness doesn't impede the devil's darkvision
So the answers for darkvision above would apply, with the caveat that magical darkness does not impede the darkvision.
Darkvision and Devil's Sight
3. Do the rules have anything to say, or is there guidance from closely connected material or other editions available?
The RAW from the 5e core rule books actually cover the questions you ask. I can find no other guidance specific to your questions in 5e supplementary material. As stated elsewhere, rules from other editions are not a reliable guide.
To list the sources defining darkvision that I have found in the core rule books:
- PHB p.183 Darkvision
- PHB race darkvision descriptions (e.g. p.20 dwarf)
- Monster Manual p.9 "monster" specific darkvision
- PHB p.108 Warlock's Devil's Sight
- Monster Manual p.70 Barbed Devil (and other devils) Devil's Sight
- (Sage Advice on Devil's Sight)[http://www.sageadvice.eu/2015/11/18/devils-sight-vision/]
And there are also rpg stack exchange questions on this, as referenced above:
- what is the correct interpretation of darkvision
- how should a warlock's Devil's Sight work
Best Answer
It depends a lot on how closely your game sticks to RAW. Under RAW, everyone who can't see will attack and defend with disadvantage. If neither the attacker nor the defender can see, the disadvantage for attacking blind and the advantage for attacking someone who can't see you cancel out and the darkness doesn't actually do anything to most attack rolls. Unless a character is using the Hide action, darkness doesn't actually conceal their location, so there is no need to guess where enemies are or attack squares blindly (sometimes DMs will house rule this). So combat will mostly play out as usual despite no one being able to see.
One side effect is that any extra sources of advantage/disadvantage (like ranged attack in melee) get blanked because multiple sources of dis/advantage don't stack. Another effect is that attacks of opportunity no longer happen, because they require vision.
Darkness does have a big impact on spellcasting. A lot of spells require vision to be castable (notably Counterspell). This can be useful for shielding the party from enemy spellcasters (although note Fireball doesn't require vision, and can be devastating to a party huddled inside Darkness). Your party's other spellcasters may want an angle where they can step outside the Darkness to cast their spells, so keep that in mind when you position the Darkness.
Conclusion: RAW, the main impact darkness will have on your party is interfering with spellcasting (which can be tactical, or annoying) and blanking advantage (like Barbarian's Reckless Attack). If your DM is using custom rules for unseen characters, such as automatically Hiding them, then Darkness is much more likely to be an annoyance.