I started playing D&D with AD&D 2e, circa 1998.
Mechanically, I didn't even like it at the time. Almost every gaming group I played with had to heavily house-rule it to make it playable for any serious or long-term game, because the game had so many arbitrary, straitjacketing rules. Many of the house rules groups came up with were precursors of changes that were implemented in 3e, which makes me think that those groups weren't the only ones seeing those problems with the game. I think I played in one campaign that tried to use rules-as-written AD&D 2e, and even then the limitations were clear.
I NEVER got THAC0, ever. For every character I played I had to write out a full to-hit chart for it to make sense, and updated the chart on my character sheet every time I got a THAC0 improvement on my character. Ditching that to-hit system was on the long list of reasons I went over to 3e with gusto when it came out.
Also, I found the obvious sanitization of the game to placate "moral guardians" concerned about D&D rather annoying, like the assumption in the core rules that Clerics worship just generic "good" or "evil" and that even having specific Gods in the game was essentially an optional rule. In every 2e gaming group I knew, the "Generic Cleric" of a nonspecific Generic Good God was something of a running joke for an unimaginatively designed stock character.
There were definitely things I liked about AD&D 2e though, things I miss. . .
It was an era of expansive, lush, rich campaign settings, a vast multiverse of D&D. TSR made so many settings, and most of them pretty well done, and interlinking them via crossover settings like Spelljammer and Planescape, into one vast D&D multiverse. It was a broad spectrum of flavors, game styles, and sub-genres of D&D. There were so many manuals and supplements produced, making a vast amount of setting lore, sub-settings, and raw information that you could spend years just learning more.
To this day, I still often go to my AD&D 2e sourcebooks for information and lore. The Forgotten Realms and Planescape books for 2e are still stuff I love to go to. The illustrations of various clergy in the Faiths And Avatars series is still really good reference for describing or showing what the faiths of the Realms look like.
Aurora's Whole Realms Catalogue. Officially made as a Realms product, but overall an awesome D&D book in general. It's the absolute best "book of stuff" for D&D. It's not about weapons and armor, but the zillion other everyday things that you might want to shop for in a D&D world, in a very period-looking book that intentionally evoked the look of a 19th century Sears Catalogue, with prices and descriptions for everything you could imagine. Even 30 years later they still have never even come close to topping it as an equipment guide for D&D.
(To be fair, the various supplements were of wildly varying quality. . .the Complete Handbooks, for example, ranged from the nigh-useless Complete Priest's Handbook, to the notorious broken cheese of the Complete Book of Elves, which had interesting lore for playing elves, but the rules elements were basically "elves are better than you and always win at everything", but when they shined in 2e, they shined bright)
Also, there was a lot of support for quasi-historic gaming. The PHB itself used historic and mythological figures as examples of the character classes, many sourcebooks presumed the game was being played in a setting that was very much like medieval western Europe with only some superficial changes. . .and there was even a whole sourcebook series devoted to adapting D&D to outright play in specific historic eras. I ran a few games in those eras and a whole campaign set during the Crusades with the Green Books, and have always regretted that D&D has focused less on quasi-historic gaming over the last 20 years as it embraced a more high-magic spellpunk/dungeonpunk aesthetic.
