Excellent post, agree with almost all of it.
Two notes, though:
1.) Thunderwave is not as purposeless as you assume, the push can propel multiple objects, which can do damage when colliding with creatures or other objects. Someone demonstrated one-shotting the Adult Red Dragon in the Githyanki patrol by using Thunderwave to push a shit ton of objects down from the bridge and on top of it.
2.) Your explanation of how Darkness is supposed to work is actually incorrect. Darkness explicitly mentions that creatures with darkvision cannot see through it, so BOTH attacker and defender are functionally blind when attacking each other, neutralizing any disadvantage and advantage on attack rolls.
Thunderwave is not useless by any means. I only noted that, alongside other mechanics in the game, its value had been decreased in relation...
As for Darkness, I'm sorry, and I don't wish to come across as harsh or aggressive, but you're mistaken; my description is correct, and I am 100% certain of that.
Darkness, the written spell, says that it creates darkness. It notes that it is magical darkness - and specifies, to support that, that
creatures with darkvision can't see through it. If it made creatures inside the darkness functionally blind, it would say so. This is a specific-beats-general classification: it is calling out a specific case where the general rule of darkvision in relation to darkness is overruled. Normally, creatures with darkvision would perceive "darkness" as "dim light". For this particular magical darkness, that is not so - it's still darkness to them.
That is what the spell says. For what effect that has on characters, we then have to look at what darkness does, and for that we need the rules on light and shadow. I'll quote them here:
(Italics mine)
Darkness creates a heavily obscured area. Characters face darkness outdoors at night (even most moonlit nights), within the confines of an unlit dungeon or subterranean vault, or in an area of magical darkness.
A heavily obscured area - such as darkness, opaque fog, or dense foliage - blocks vision entirely. A creature suffers from the blinded condition (see appendix A) when trying to see something in that area
Also, the wording on the Darkvision trait:
... 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.
Note that it is not talking about where
you are, or what condition you are in - it's talking about what you're seeing; a space apart from you but still within 60 feet. You can see in that patch of darkness, over there, within 60 feet of you, as though it were not darkness. If you're trying to see a creature that is within 60 feet of you, and that creature is in darkness, you aren't blinded against that creature - you can see it as though it were in dim light. That's how the trait works.
The Darkness spell specifically calls out its overruling of the darkvision trait, to say that even creatures with darkvision are still functionally blind when trying to see a creature within this particular magical darkness, even if the thing they are trying to see is within 60 feet of them.
These are all of the relevant rules for 5e, and they're consistent with the way light and shadow generally, actually, works. Darkness and light are not - and are never - about where you, the eyes looking, are. They are always about
what you are trying to see. If what you are trying to see is not in an area that is heavily obscured, then it doesn't matter if you are... and if it did it would make hiding in ambush impossible.
Can you imagine the ranger attempting to ambush his foe from a dense patch of foliage... only to be told by the DM, that he has disadvantage because he is in an area that is heavily obscured? That's not how it works. Of course he's in an area that is heavily obscured - he's hiding from his foe and doesn't want to be seen! His
foe is not in an area that is heavily obscured, so the ranger does not have disadvantage attacking them... the ranger has advantage, because he is an unseen attacker. The enemy, meanwhile, will have disadvantage to attack the ranger because they are trying to see the ranger, who is in an area that is heavily obscured.
((Edit: though I will concede, that it seems like the greater majority of people and players (at least in internet threads) all run darkness incorrectly... and that it is run and ruled incorrectly more often than it is run as written and intended... So please don't take any of this as an attack))
