Great post, it's a masterpiece!
1) Alignment: As much as we like to talk about the "9 alignments" as a sacred cow, it actually was just Law, Neutral, and Chaos back then. It seems that Law was "Big L, little g" and Chaos was "Big C, little e".
I'd rather drop the alignment system from the game altogether, but this was always my favorite way to implement it. I would just add one more: Unaligned, which would be the default for most PCs and NPCs alike. Paladins would be Lawful, Druids would be Neutral, and Clerics would match their patron deity, because that's a big part of what those classes are about, but everyone else would just be Unaligned unless a PC specifically requested an alignment.
2) As we talk about bounded accuracy today, there are several places where I find it interesting how much more "bounded" the game was back then. Some examples:
a) Ability scores were more spread out. You had to get very higher scores just to get even a +2 or +3, and you didn't go above 18.
9-12 +0
13-15 +1
16-17 +2
18 +3
b) Many more things used static rolls instead of adding in ability scores. Several skills, initiative, surprise, even your saving throws were almost entirely dependent on level than on your ability scores.
c) Hitpoints were tighter. Fighters only had d8 hp, and you only gained a single HP at 9th and beyond.
Personally I much preferred that approach to ability scores. I get why we moved away from it but I think the 3e-5e system of getting an additional +1 every other point, and having those modifiers apply to many more things, is LESS needed in 3e-5e due to skills, feats, subclasses, and other customization options that weren't around in previous editions.
And hit point progression plateauing at "Name" level was like the E6 (I guess it would be E9?) of the old days! I'd like to see it brought back.
9) It was interesting to read the "Mystic" which is the original monk. The monk honestly hasn't changed nearly as much as I had expected, and many of its current abilities you can see traces of in the original class.
Still the best name for it IMO!
18) Initiative was very different back then. It was a simple d6 and done by each group. The ideas of adding dex to the roll and rolling it per person were actually optional variants at that time. So was surprise, there was again no perception check back then just a simple d6 done by both sides. I like the simplicity of it, but considering how deadly surprise can be its probably a good idea they changed it.
19) The Monster Reaction and Morale tables are actually very simple and yet I really like how they make encounters more organic. Monster reactions showcases things like animals that may not be hostile due to certain circumstances, and morale gives you reasonable "checkpoints" on when to consider if a monster should just leave a fight. Its very clear that back then, it was more common for monsters to leave the battlefield than to just get killed.
20) Weapon Speed did exist in a very simplified version: Ranged Attacks went first, then Spells, then melee weapons.
I love all of these and hope they come back in some form! Initiative by side, with weapon speed determining order, was elegant in being simple yet still encouraging the PCs to coordinate in battle.
And not only does a morale system just make sense, even in a fictional fantasy setting, it goes really well with XP-for-treasure. I'd like combat to be an interesting and an assumed part of the game, yet still have it be an obstacle to the ultimate goal rather than the goal itself.
23) THAC0 tables really are as nasty as I remember
The one change I would make to any of the editions prior to 3e.
25) But at high levels, the opposite is true—high-level characters almost always make saves, and this didn't change until 3rd edition borked the saving throw math for ever afterward. And you'll notice that dwarves and halflings hit those good save numbers way sooner than everybody else, followed by fighters next—fighters being able to make most saving throws is one of their hidden bits of awesomeness (along with their use of magic swords) that makes a "plain" looking class actually rock the house in Basic when played rules-as-written! (Even if you don't use Weapon Mastery!)
Great point, this is probably part of why BECMI/RC Fighter always seemed the least generic to me. As in all editions it was still a simple class, but out-of-the-box I felt it had more flavor than usual.