"Dictums are given for the sake of the game only, for if ADVANCED DUNGEONS & DRAGONS is to survive and grow, it must have some degree of uniformity, a familiarity of method and procedure from campaign to campaign within the whole."
"If it is all too plain and too easy, the players will quickly lose interest, and your effort will prove to have been in vain. Likewise, if the campaign is too difficult, players will quickly become discouraged and lose interest in a game where they are always the butt; again your labors will have been for naught."
"The danger of a mutable system is that you or your players will go too far in some undesirable direction and end up with a short-lived campaign. Participants will always be pushing for a game which allows them to become strong and powerful far too quickly."
-Gary Gygax, AD&D Dungeon Master's Guide, Page 7
The huge push for AD&D 1E had two parts: D&D's early game was going in a million directions and getting out of control, and business reasons (royalties) to consider when creating a new game. That last part sucked, but I get it. But Gygax's words at the front of the original DMG warned us against much of what came after.
One is a warning against BX. I get it; BX is an amazing creative platform, far better than AD&D, and people knew that back in the late 1970s when this all started to come apart and AD&D was needed. Go to a convention, sit down to play "D&D," and you don't know what you're playing. You see this today with 5E tables, and the push for standardization is strong and needed for organized play.
Another warning worth heeding is making the campaign "too plain", which you see today in the multiverse-of-magic model modern D&D 5.5E is built on: every world is a colorful box of marshmallow shapes, everyone has magic, the planes are everywhere, and magic replaces technology. Today's modern, cosmopolitan, sci-fi, alien-packed D&D is both too mundane and far too magical. No unique ancestry is special anymore. Magic is so common it is boring.
A power level that gets out of control kills your game and drives players away. I don't like DMing a game where players laugh at carefully created challenges, and D&D 5.5E is horribly broken above 10th level, and they admitted they slacked off on testing the game's balance. Gygax carefully balanced AD&D to work all the way up to high levels. Ever since AD&D 2nd Edition, they have been destroying that.
But compared with BX, AD&D gives me more mental stimulation and puzzle-solving, which I enjoy more than BX's straightforward, highly malleable game. BX is like Silly Putty; it can be anything you want: a sci-fi game here, a detective game there. Where BX falls short for me is in being too simple and too malleable. There is nothing to "sink my teeth into" regarding weapon speed, reach, and encumbrance playing a major role in my martial character's survival.
There is a sidebar here about "BX Incorporated" which feels like a lot of the OSR these days, and the non-BX options feeling like second citizens. Gary's feelings about BX being so malleable and undefined are both its greatest strength and fatal flaw. I love BX, but not every retro-clone needs to be it, and we've forgotten plenty of what made AD&D so great.

My BX mage can choose from infinite spells and understand them all instantly. In AD&D (specifically OSRIC 3), I have a maximum number of "understood" spells per level, which defines the pool I can memorize daily spells from. Sure, I can play an INT 9 magic-user, but I can only understand 6 spells per spell level, and I will need to swap them out and relearn them from my spellbook to put them into that memorization pool. Practically, your INT is high enough that you don't hit the limit unless you are very high level and have collected a few hundred spells. Realistically, this is the balance we have always needed for casters: it forces them to study and build spell pools to pull from when it's time to memorize daily spells.
I like this feeling for magic! The spells are so big and complicated that they're hard to hold in your head, and you need to keep studying to swap things out and pull in ones you haven't used in ages. This one rule creates the Gandalf who sits by the fire, reads his spellbook, makes room in his head for that wall-of-ice spell he never uses, and tells the others to be quiet so he can get his understanding to roll, so they can all live tomorrow.
In BX and D&D 5E, mages have infinite RAM in their memories and can hold 300 first-level spells in their brains, plus 1500 others. There is no "re-understanding" a spell, and mages become entitled and overpowered mana conduits of death, with far too much spell choice, no forced thematic study, and, frankly, boring and overpowered. BX at least makes them less survivable, but I love AD&D's quirky thematic rules that force a magic-user to "act like Gandalf."
And games that copy elements of AD&D but forget these Gygaxian fiddly parts emulate the math but miss the finer details that "make the dish complete." The new OSE adopts AD&D-isms, but it omits all the intricate, Swiss-watch, deeply thematic, genre-defining minor rules. Watered-down AD&D is sometimes like a fast-food restaurant trying to make its own version of what a 5-star chef would prepare.
Better than the burger on the menu? Yes. Is it exactly the same as what the master chef gave us?
Clearly, no.
And AD&D 2nd Edition, a game I love, lost its soul and started to creep into story gaming, moving away from the fiddly, intricate mechanics that forced players who learned the rules to act like the class the rules presented.
And OSRIC 3.0 is the community-supported version of the game, always there, and easy to learn and teach. People can publish for this. Until Wizards puts the original books back in print (unchanged) and open-licenses them, this is the best we got. You can't touch perfection. This is as close as we can get, and it's something we can build a community around.
And AD&D/OSRIC is far less complicated than D&D 5E, especially when you factor in the action economy, how builds interact with it, and what's allowed and what isn't. Mid- and high-level D&D 5.5E play is more complex than GURPS, and the combat turns take far longer to figure out. With AD&D, the mid-complexity is harder than BX and low-level 5E, but it keeps a flat difficulty curve all the way up to level 20. BX stays too simple, while 5E gets far too time-consuming.
And the complexity pays off, especially for martial characters balancing weapon speed and reach, picking specializations, gaining those bonuses, and fine-tuning their combat profile. Oh, and if you are not wearing a helmet, roll a d6. On a 1, your AC is 10 this turn as the blow comes at your head. Ah, thank you; all those dyed Supercuts hairstyles in 5.5E with characters missing head protection, go away. Football players and soldiers wear helmets, so should you. BX doesn't do that unless house-ruled, but why house-rule when we have it here, and it is a standard rule at all tables?
The more I sit down, forget the biases modern games have implanted in my mind, read the original AD&D books, and learn the classic 1E game, the more I see what's wrong with today's dichotomy between BX and 5E.