- Descending AC and THAC0. (My impression is that most OSR games have abandoned this.)
- Nonhuman races have level limits.
- Emphasis on domain-level play after 9th, divine ascension after 30th in Classic.
- Preparing spells took much more time, and a high-level spellcaster who had cast everything might take days recovering.
- Magic items were vulnerable to area attacks and could easily be destroyed.
First off I note that your favourite era was my favourite era of D&D also. Though I did enjoy 4E and enjoy 5E also (I did not enjoy 3E, overall).
I think these are the weakest identifiers for how AD&D was actually played, at least in the 2E era. I also think there's such a stark difference in how people who started in the 1970s and early 1980s played, and people who started in the late 1980s and 1990s played that it's more like there are at least three eras here. The very dungeon-crawl-y, gritty, sometimes monty-haul, sometimes just mean-spirited era of 1E, and the more role-playing-oriented, more story-oriented, less dungeon-centered play of 2E, before 3E, where mechanics took center stage more than they ever did before.
THAC0 was trash. No rules-related moment in 2E was better than when, in the late 1990s, we did was a lot of groups were doing, and simply flipped THAC0 and AC so it was an attack bonus and ACs went up. This had absolutely zero impact on how the game felt, atmosphere-wise, but it made game run faster, and make more sense to people.
Nobody actually enforced racial level limits. I mean, I'm sure someone did, but I played with quite a number of groups, and spoke to people on the early internet about this a lot, and it was not something people actually enforced. Particularly as they were levels rarely reached and if the High Elf does hit his cap as a Fighter or whatever, are you seriously going to say "Well that's that for you buddy!" whilst the rest of the group keeps leveling? You are not going to say that. You are going to make an exception, which is what people did. Or a Wish or or divine blessing or something will get involved.
Domain-level play? In 1E I can believe it, but in 2E? Nah. As Quentin said, people didn't actually play it like that. It was nice to have it as an option, and the idea was not unsound, but it wasn't really a thing in any 2E group I played in. The design of 2E settings other than the admittedly-wonderful Birthright tended to reflect this.
Preparing spells taking forever also didn't get enforced in 2E. I suspect most groups weren't even really aware of the rule.
Finally, magic items being vulnerable was one of those things that only got remembered when it was dramatically convenient for either the DM or the PCs. The DM would suddenly realize actually, he could destroy a bunch of items he foolishly gave out, or the PCs would be like "Well we fireballed that dude, so his staff should be toast!" (even though they'd be appalled if the same applied to them). That was definitely not consistently applied that I ever witnessed.