I see someone's forgotten about weapon speed factors, weights measured in gold pieces, modifiers based on opponent's armor class, nonweapon proficiencies with different numbers of slots required, ranged being measured differently based on environment, round vs turn distinctions, and any number of other things.
You got me. I don't think I ever played AD&D, but I did read the 2nd edition rulebook, and it struck me as a patchwork where every ability had its own custom system that didn't look like any of the many other patches of system in there. One thing 3rd did was clean it up and reduce it to one simple, big over-arching system that handled everything.
It's just that that one system had a lot of moving parts. Every point of AC had to come from somewhere, everything had to be typed, and there's absolutely a lot of logic behind it, but instead of just AC, you've got touch AC (without the armor bonus), flat-footed AC (without the Dex bonus), etc.
They have different kinds of complexity. One is a patchwork of countless little subsystems that you can never all remember, and the other has one really big system that does everything.
Though the real complexity in D&D 3 is in the builds. A whole industry sprung up around figuring out new broken builds, what classes to dip for what bonus, etc. I think the easy multiclassing was a mistake.
(Despite having played RPGs since 1984, D&D 5 was the first D&D that I ever bought. I was mostly playing other systems.)
I see someone's forgotten about weapon speed factors, weights measured in gold pieces, modifiers based on opponent's armor class, nonweapon proficiencies with different numbers of slots required, ranged being measured differently based on environment, round vs turn distinctions, and any number of other things.