Functionally, THAC0 as seen in 2e is exactly the same as the additive to-hit calculation we've had since 3e. When attacking you want to roll high and have as high a positive modifier as possible.
To hit formula:
[To-hit bonus on character sheet] + [dice roll] +[modifiers] = the AC you hit
THAC0 formula:
[THAC0 written on character sheet] - ( [dice roll] + [modifiers] ) = the AC you hit.
We see that in both cases the final result scales linearly with the dice roll and the modifiers. So why is THAC0 harder to use than To-Hit?
- Doing both subtraction and addition involves more of the brain than only addition, especially once you have double-digit numbers
- With To-Hit, you can add the numbers you have in any order and get the correct result, but since THAC0 involves subtraction you either have to follow the exact order of operations in the formula, or you have to keep track of when to flip the sign.
All in all, THAC0 is just a more complicated way to reach the same result, with 0 benefits.