THAC0 isn't hard to explain. The explanation is easy. The trouble is that It's harder to
execute. Like the fact that you have to stop and read these to figure out which one is right is part of the trouble:
- THAC0 - (die roll + modifier)
- THAC0 - (die roll - modifier)
- THAC0 - die roll - modifier
- THAC0 - die roll + modifier
Doing it in your head correctly when sometimes you have a circumstantial -1 or +2 or -2 or +1 that varies from round to round and that sometimes you forget until after you think you're done is not easy. Worse, because AC basically "reflects" around 0, it's easier to miscalculate without noticing. If you get a result between -3 instead and 3 as the AC you hit, that generally feels always about right with THAC0. It's harder to look at your die roll and say, "wait a minute, that doesn't make any sense because it's lower than the die roll".
THAC0 isn't difficult, but it's
precise and that makes it
cumbersome. It's just as cumbersome as the attack routine tables from AD&D 1e. That makes it error prone in ways that adding numbers isn't. Because it's literally the most common roll in the game, being rolled as often as 3 or more times in a single round every round, having the mechanic be even slightly more complicated than it absolutely needs to be is a failure of design.
Remember: Nobody ever made a THAC0 wheel for 5e.