Bacon Bits
Legend
Those were good days. Back when D&D was about players, and not endless lists and silly traits and abilities. THACO wasn't a bad idea, but it wouldn't carry forward ;it assumed a gamer base that was accustomed to doing math in their head*. Styles have changed.
I played brown box and 1e a lot longer than I played 5e.
*= 'You won't always have a calculator!' Little did anyone know....
It's not really about "doing math in your head" being something older gamers can do and newer gamers can't. If anything, the older gamers still using the system are merely those who happened to be good at this particular calculation not math in general. The fact is, the math is more difficult than it needs to be, and that inherently slows the game down because you're doing it very often. Between the PCs and DM, it's not unusual for a table to make 20 attack rolls over the course of a single combat round. So it might be just a little bad, but when it has that much of an impact on the game being a little bad is a big problem.
Here's the real problem:
I give you three numbers. One is your attack roll, one is your THAC0, and one is a situational modifier. Given in no particular order: 5, 8, 3. What AC did you hit?
Now do the same thing for 3e attack rolls. One is your attack roll, one is your attack bonus, and one is a situational modifer. Again, in no particular order: 5, 8, 3. What AC did you hit?
With the descending AC system, you can't answer the question. You don't have enough information. The mechanic is immediately more complex than "just doing math" because you have to know not just what the formula is, you have to know what each value is and plug them in correctly. It gets even worse because if you do the math wrong, you often can't tell. A wrong answer will sound correct because it'll be in the correct range of possible answers. In truth, with the descending AC system, you're not just doing addition and subtraction. You're doing algebra. You're doing algebra a dozen times every few minutes with both random and varying numbers. It's a horrifically poor system that was originally partially designed to obfuscate outcomes (the attack tables were DMG information). After that changed due being totally obnoxious it only serves to encourage errors. It is not a defensible design under any amount of scrutiny.