It can't be the AC system. "positive/additive" AC is just plain superior to thac0, end of story.
AC was wonky, but not really difficult. It was largely static unless you upgraded gear. Thac0 was also wonky, but made sense in the context of providing a formula for the older attack matrix. But since it didn't really change much (only at levels or with magic items uusally) without the constant modifiers found in newer systems, using Thac0 wasn't really that difficult.
It can't be the absence of feats, because 5e can be played without them.
5e can be played without feats, but the classes and rules are still much more complicated than in 2e. Keep in mind that feats really only exist to give options for customizing your character even further, yet the character customization has been baked into the core classes in 5e already. The argument made sense for 3e, where feats were the only non-equipment/non-spell way of customizing the character, but that doesn't hold water in 5e.
It can't be the skill resolution system because let's face it, 5e is simpler and better.
2e skills were not only entirely optional, but still much simpler even if used. With 2e, most rolls were done as a ability checks (roll less than or equal to your ability to succeed) or simply not required in the first place. Exceptions involved things listed in the ability block (bend bars/lift gates, for example), and of course skills if you chose to use them.
It can't be the magical items, because 5e brings the old school back and the atunement rule is superior - but if you don't like it very easy to remove.
I like the way 5e handles magic items, particularly in how they are supposed to be somewhat rare. 2e sort of assumed that players would have some, but not that many. Melee classes tended to have more options due to armor and weapons, and it was always nice to find a ring of regeneration or something, but you could easily go 10 levels finding nothing more than a longsword +2.
The two systems are very similar in that regard, with 5e just enforcing it a bit with attunement.
It can't be the ability scores because 5e is much more regular and "fairer" - a 13 is worth something now.
Agreed, to a point. Ability checks used to be much more common than they are now (since skills weren't used or had over-specific purposes), so having an ability of 13 vs 12 still provided the same 5% increase to succeed. That said, I do like the 3e+ system where all abilities simply confer a bonus, and feel like that's a stronger mechanic than what came before.
So what is it? Is it the multi-classing? Bounded accuracy? The absence of warlocks, barbarians etc? The saving throws? The less HP? The initiative system? Spell disruption? No cantrips? what?
For me, it's the fact that the system demands players actually roleplay first and foremost. There wasn't much in the way of min-maxing a character back then, outside of having unrestricted access to magic items, which meant you needed to differentiate your character in ways that weren't represented by stats. You'd often have a group with multiple fighters (because the stat requirements were fairly easy, and fighters were also important), and they'd each generally gravitate towards equipment types and personalities in order to stand out. That was fun, because people actually desired to play up the roleplay aspect of D&D, even when the bulk of adventures were just dungeon crawls.
It's also worth pointing out the major design changes that happened with 3e, and *why* they happened. D&D (up to and including ad&d 2e) was not a rules system intended for computers. It had funky positive/negative, roll low to succeed kind of mechanics, largely as a result of early editions using charts and tables (matrixes) and also because it was just expected that the DM was the real source of power and control (a very Gygax way of controlling the group). By the mid 90's video games were starting to really take hold, ultimately culminating with the release of Baldur's Gate.
Baldur's gate was a great game, commercially and critically, but it was no secret that the 2e rules system did not translate well to a digital game. Spells with vague mechanics had to be clarified, and class abilities that normally relied on DM fiat needed changing (rogue stuff, especially). Not to mention game calculations involving Thac0 and saving throws didn't port well, and required several computational steps to solve for each.
It's no accident that when 3e came out next, the system had a much more computer-friendly set of mechanics. That was a good thing for games, and a good thing for people looking to min/max stuff, as well as introduced concepts like "character builds" into d&d.
When you find someone that really loves 2e, odds are they love it for reasons that have little to do with how good the mechanics were, and instead with how the mechanics left enough holes for the DM and players to fill with stuff that made their campaigns and characters unique and fun. AD&D 2e was simply the last version of D&D before the paradigm shifted more towards a rules-complete game system rather than a roleplay system.
