2e is what I cut my teeth on and will always be my favorite edition. It has so much incredible lore and worldbuilding (as does 3e) in the mountain of content available for it and the art is some of the best that has ever been painted for D&D. For those of you that haven't visited the D&D thread in the tabletop forum, you won't have seen the photos of my absolutely fucking ridiculous D&D collection. I've got more than 500 hardbacks from 1e to 5.5 and triple that in softcovers, boxed sets, etc. I also have a physical copy of every Pathfinder 1e adventure path. It's absolutely retarded how expensive some of those 96 page softcovers have gotten.
I have everything ever printed for Dark Sun, Planescape, Al-Qadim, most of Ravenloft and The Forgotten Realms (there's just so much for this setting and not all of it was good). I have a few things for Dragonlance, but not much because it's an amazing setting to read about, but not so much to play in. Almost nothing for Birthright and Eberron because neither of those settings ever appealed to me. I always thought Spelljammer was gay so I don't have a single piece for it and the other super small print-run settings like Mystara.
Not bragging -- just pointing out that I'm a superfan. Although I wish I could turn off my collecting habit since the entire TTRPG industry has become terminally infected with Leftist cancer.
Anyway, regarding Thac0, it always made sense to me because I viewed AC -2 as a penalty to my attacks because the NPC was heavily armored/difficult-to-hit and that's exactly what negative AC was supposed to represent. I don't think AC being negative was ever the real point of contention. The thing that confused people a little is that each character's Thac0 had its own scale based off class/level.
For example - A Fighter's Thac0 improved by 1 every level and a Wizard's improved by 1 every three levels. Rogues and Clerics each had their own scale. Still, it wasn't something super archaic. Your character sheet had a field called Thac0 and you just updated it when necessary. Just like you do for Hit Points or Proficiency Bonus in 5e.
I genuinely believe that people who were (or are) confused by Thac0 are legitimately dumb because no one that ACTUALLY played D&D 1e/2e was confused by it. There has always been a shitload of people who know what D&D is (and may have even bought some of the books), but never really played the game and I'm almost positive that they're where most (if not all) of the confusion stems from.
TL;DR -- if you play(ed) D&D 1e/2e and you're confused by Thac0, you're probably a potato, but if you haven't played 1e/2e -- understandable.