I need to preface this post with a disclaimer. I love 2e. I played it for a very long time. But it has warts, and whether something is a bug or a feature is often in the eye of the beholder, so to speak.
2e is a very different game. It has tons of splatbooks to draw from, potentially giving you a bewildering array of options that is at least equal to the 3.5 era. You don't have to use them all, of course, but I played in a few campaigns where "if TSR wrote it, it's legal for use" and the disparity between a guy who plays a Fighter from the PHB to someone who is a specialty priest out of Faiths and Avatars, has psionic wild talents, a kit, and has a cracked-out race from outside the PHB like Sylvan Elf, Tiefling, Aasimar, or something from the Complete Book of Humanoids, is telling.
You have Elven Dualist Wizards, Wild Mages, Viking Rune Magic, Spellfire Wielders, Psionicists, Quest Spells, Elven High Magic, Elementalists, Sha'ir, and probably seven or eight even more esoteric powers I'm forgetting that are potentially available to players. The game can run from magic-high (Forgotten Realms), magic super-high (Arcane Age), magic-low (your typical Ravenloft game), and weird hybrids like Dark Sun (magic super low to Dragon Kings).
Just about every rule in the PHB can be bent or broken in later books (my Paladin can't have weapon specialization? I can't dual-wield long swords? I can't be a multi-classed Bard? Hold my ale!), and only once did TSR ever try to rebalance things (Complete Priest's Handbook)- most books are pure power creep. So you can embrace the gonzo madness, or try to lock things down to the bare essentials, as you like.
Character classes aren't balanced against each other, even if the different xp tracks might lead you think so. Some classes are useless until about level 5. Thieves can't reliably do anything at low levels (even with the best possible race/Dex combo in the PHB, it's going to take til level 4 to get to 95% hide and move silently, as examples, and you're going to be terrible at anything else).
Warrior classes are beasts for a good while, with ever-increasing saves, great hit points, and reliable damage....then one day out of the blue, spellcasters are able to break reality in half...or not, since "save neg." relegates many spells to worthlessness, and the game isn't shy about giving out immunities and magic resistance like candy.
Everything is slave to RNG- your character can be puny or godlike based on random chance, and random chance can fell even the most powerful character, forcing many to try and make ever stronger characters.
And you will find out quickly that, despite what the books claim, you can't just make whatever character you want- system mastery permeates the game, from what ability scores are best for you, to even what weapons you should use (longswords yes, two weapon fighting, yes, two handed weapons, no).
And finally, well, about Thac0, I'm not going to weigh in on whether it's good or bad, but instead I'm going to bring up how horribly it's presented, thanks to D&D-isms that were present from day 1.
As you level, your Thac0 goes down. And your AC goes down. But rather than subtract from Thac0, your ability scores and magic items increase your die roll. Your Dexterity defense mod. is a negative value, which makes sense, since it lowers your AC. But just about any other beneficial modifier to AC is represented as a positive value!
I've had new players express confusion that somehow a Shield +1 isn't a cursed item!
Combine this with the fact that you want to roll high on saving throws and attacks, but low on ability checks and Non-Weapon Proficiencies...and other times, rather than use a d20, the game wants you to use d%, and it's more than a little oblique to new gamers.
Oh and about those Non-Weapon Proficiencies- it's always interesting that the more books you use, the less characters can do without proficiency! Soon you need proficiencies to notice things, to observe things (not the same!), to fast-talk, intimidate, gather information, loot treasure quickly, beg for coin, mimic the sounds of animals, guide a boat down a rapid stream (no, sorry, Seamanship doesn't apply!), Acting, seduction, pretending to be asleep, walking on water, or even (a personal favorite), Giant Kite Flying!
All in a system that gives most characters 3 proficiency slots at level 1 and a new one every 3 levels! Oh sure, maybe your Class (Bard) or Kit gives you a few free ones, and you can always choose to know less languages, but there's no way you're going to get more than a few of these, and the system for improving proficiencies is, well, pretty draconian, to say the least. Oh and some proficiencies cost more than one slot!
(And while the PHB claims that proficiencies are optional, that flies out the window with the very first splatbook, lol).
TLDR: 2e has a great deal of charm, gonzo insanity, and wild ideas a plenty. It's also a hot mess of a system. If you're familiar with all of it's idiosyncrasies, you probably love it. Or hate it.
Most likely, a mix of the two. It's likely the most house-ruled version of the game that has ever existed, to the point that any two random players will have incredibly different experiences playing the game.
When it was good, it was and incredible, amazing experience. When it was not good, it was wretched, bitter experience.
And I haven't even gotten into Dragon Magazine content or the "Player's Option" books yet!
