As much I like to grumble Thac0 Kept the Riff-Raff Out! I’ll grudgingly admit it kept out a lot of potential gamers, too.
Wizards of the Coast was right 20 years ago, attack bonuses and ascending AC are more intuitive compared to Thac0. Their marketing and the new rule itself won D&D players over.
Then came the modifiers. Lots of modifiers to that seemingly simple attack roll (along with other checks). If you are a player, you used skills, feats, spells, and whatever situational modifiers you could devise to pile on those bonuses in your favor. Stack those bonuses high.
Thac0 certainly wasn’t a game breaker (unless your had a negative 4 or less Armor Class). Yet those modifiers in post-2000 D&D, mein gott im himmel! When you’re high level, rolling +15,+20, or +25, should the d20 even matter anymore?
Eventually my poor DM head broke trying to keep track everything. D&D 3.5e became a chore to run when player-characters reached 10-12th level. I once ran a combat with six 15th to 17th level PCs against a group of evil monsters and NPCs which lasted 4 hours. A single encounter.
D&D Fourth Edition made every encounter last an hour and modifiers got worse–especially when multiple PCs possess area-of-effect powers which went from round-to-round.
Now I’m not certain which kept more people out, or which caused more burn-out: Thac0 or all those modifiers.