The spell list. Or specifically, having magic be a bunch of blocked rule texts that's separate from other forms of resolution. Any sort of skill-based or freeform magic makes the game into "not quite D&D".
Strongly disagree. In a memey sense I'd be all like:
Any sort of skill-based or freeform magic makes the game into "not quite D&D" "BETTER".
"Fixed that for you." But that'd be kinda rude, so, I went with the first approach of "strongly disagree".
You're describing what I find--as a professional game designer, and being broke and penniless, desperately seeking any crap job I can get so I can take my dog to the vet, it's one of my few joys in life that I can honestly refer to myself as a professional game designer, so please don't take it away from me lol--to be the single worst aspect of the game as being the most essential aspect of the game.
My gold standard in many ways is HERO System. First, I'll recognize that it is extremely niche and not for everyone due to its lack of any implied setting outside the setting specific books, and more importantly due to its incredibly high crunch which scares away a lot of people, especially chargen which is so complex it requires special software for most people to complete (I can build a HERO System PC at 200/150 purely on paper from the book, but that's an ability I'm proud of, like a "stupid human trick", it's not something I'd want to ask of the average gamer if I could avoid it).
Anyway, in HERO System, if you can get past and through the initial wall of incredibly high crunch, you can observe that virtually every single power in the game works on the same basic principles using the same basic procedure. Resolving an Energy Blast or hand to hand attack involves essentially the same process as resolving an Ego Attack, a Presence Attack, Telepathy, Mental Illusions, Ability Drain or Mind Control. The game is complex but it is extremely consistent in that once you know how to do a thing in HERO System, you more or less know how to do all the things in HERO System.
Still needing to constantly reference special snowflake spells is probably the single thing about the current edition of D&D that I like the least.
I think it's very possible to design a version of D&D that is still very much D&D but has this kind of consistency of rules. I also think it would be a very difficult and time consuming task, so much so that even I would not want to take it on for free.
What do I think is the essence of D&D?
Grown people shouting very loudly and excitedly at a die that either came up a natural 20 or a natural 1, or failed to come up on the right number. JK
Good quesiton. There are all many, many things I think are the essence of what D&D is fundamentally about, individually and/or in combination. Here goes:
For me, D&D is about exploring exotic locations--often dungeons--searching for fabulous treasure, and fighting monsters--often dragons--that are guarding it. I'm reminded of my favorite Army slogan: "Join the army and travel around the world, meet interesting people, and kill them." Replace people with "creatures" and that more or less summarizes D&D.
Like Morrus said more succinctly in the very first response, to a degree it's about your characters going from very possibly doomed novices to the major movers and shakers in the magical geopolitics of a country, a continent, or even one or more planes at the highest levels. It's about getting comfortable in your character, comfortable enough to roleplay your character's conscience/ethics/morals in a way that is a more nuanced expression of your character's alignment; it's about being a murder hobo, but it's also about how your character feels about being a murder hobo, or what justifies it to them. This has been true since several editions before it was finally codified in 5E.
D&D is about finally finding awesome magical treasure...and equally about bickering over the division of treasure with other party members.
D&D is about leveling up and getting psyched at the new stuff you can do at the new level.
D&D is about the real possiblity of dying anti-climactically to a random encounter, an unavoidable trap, and/or bad dice luck...and about whether or not the other PCs can/will get you
raised.
D&D is about avoiding getting TPK'd, which means TPKs need to mean, or at lease seem, possible.
As a DM, the essence of my personal joy in D&D is to design encounters or scenarios that leave the PCs feeling absolutely doomed to failure and death only to barely squeak through and survive to be victorious, because of their character abilities, cunning and unpredictability, not my fudging. When the PCs exchange crisp high fives after surviving something that I knew was survivable but made seem unsurvivable via various tricks, that is the sweet spot.
Most importantly I think, it's about pretending to be Elves and Dwarves, Fighters and Wizards, Dragonborn and Warforged, Warlocks and Rogues, and dealing with their problems for a few hours every week during which you don't have to worry about your own, probably much less fun, certainly less interesting, problems.