I think one of the big mood shifts, that happened sometime between 3rd and 4th, is the stripping away of consequences from player choices and actions. You can be any race, any class, put your ability scores anywhere you want, specialize in any type of magic, and still be competent at everything you set out to do. And if your low-level character has a weakness, then soon enough you can take a feat or a subclass that shores it up. Likewise, the use of magic is largely free of consequences. Spells are never dangerous to the caster, and never powerful enough to hurt a player's feelings by denying them the thrill of combat for more than a round or two. 5e combat feels more like a Champions slugfest than a (pseudo-)medieval battle.
First edition was all about niche protection and consequences. It was arguably too restrictive (e.g. dwarves can never be magic-users) but at the same time it gave a reason to experiment with different classes and tactical approaches to the dungeon.
So if I wanted to AD&D-ify 5th edition, I would try to do a few things in addition to many of the ideas mentioned above:
1. Give the different species real advantages and disadvantages compared to the human baseline, without restricting class choice.
2. Reinforce the meaning of ability scores: Str boosts melee damage only, Dex boosts ranged damage only, Int always powers arcane magic, Wis always powers divine magic. No feats or subclasses can permit a PC to just shift all their main attacks and powers onto a single ability score.
3. Rework the whole spell list. Spells that are relatively powerful for their level should carry a cost, consequence, or risk. Some spells are "save or suck" which is why you travel as a team. Knowing such spells are out there motivates players to use tactics, not just rush in and trust to their bloated HP and spell slot pool to carry them through.