AD&D does not offer instant gratification. I create a one page dungeon with no guarantee that the players will ever go there. Magic-user spends thousands of gold to research an original spell when he doesn't even know if he has passed his "chance to know" roll for it or not.

The player has an opportunity to make a lasting impact on how the game is played. But given, say, five different spell concepts... he won't know which of them will actually enter play and become incorporated in to the campaign's legendarium.

Meanwhile, the players have to come to a consensus about what 2 do in any given session. This can lead to either conservative, overcautious attempts to extract low hanging fruit from lazy and unimaginative DMs. It can also lead to surprisingly innovative departures from the norm.

If you think you are about to get the hang of this, the death and replacement of key PCs, the leveling up of others, the discovery of magic items-- all consistently change the balance of power, shift the scope of what is even possible.

Everything is extraordinary. Everything is cast in terms of an exception to the rules. The classes, the monsters are all like this. Monsters are not bags of hit points. They are collections of rules violations. Classes even more so.

The unimaginative see an easy avenue to "improve" on the game, to "innovate". The house rules are so obvious. What to ignore is self-evident. But what the game itself actually accomplishes never seems to enter the analysis. Why does it works in spite of its "obvious" brokenness?

Well, what happens when you play a wargame scenario repeatedly-- GEV's Breakthrough for example. Early sessions are chaotic, a contest between who can make the least awful blunder. Eventually and unbeatable tactic is discovered and the chaos is replaced with a cold inevitability.

At this point you need a new scenario, new units. Drop into a raid scenario on a new map with GEV-PC's and trading in heavy weapons teams for infantry and you suddenly have to improvise again, take chances. This is exciting! There is something new to explore.

AD&D is not designed to be mastered, to be controlled, to be predictable, to be static like a worn out war game. Every design decision is selected to maintain a healthy, vibrant chaos FOR YEARS AND YEARS OF CONTINUOUS PLAY.

Almost every reaction to the AD&D system is a conscious or unconscious rejection of this chaos. In some cases, they may produce the perfect game for a small group of people. But they never produce a great game for the greatest number of people for the longest running campaigns.

The design objective of every rpg that isn't AD&D is to specifically prevent you from "building your own table" as @NotJonMollison phrases it. Intentionally or inadvertently, they create problems AD&D doesn't have specifically to keep you on a product treadmill.

What everyone sees as being OBVIOUS design flaws to AD&D are all things that directly support its design objectives. As "do it yourself" hobbyists, Gygax and Arneson operated under fundamentally different constraints from the people that need to sell you something.