I’m not an OSR gal and I haven’t played any pre-3e editions of D&D (and accordingly, didn’t vote in the poll), but I will say this for THAC0: if you tell the players their target’s AC (I know, probably not something most OSR DMs are wont to do), it allows them to do the math *first* and figure out what number they need to roll to hit. This changes the process of rolling to hit from:

1. Roll the d20

2. Add your to-hit bonus to the result

3. Compare the result to the target’s AC to find out if you hit

to

1. Compare your THAC0 to the target’s AC to find out what number you need to roll on the d20 to hit

2. Roll the d20 to see if you hit

The latter makes for a (subtly) more psychologically satisfying experience of rolling to hit, due to having a more traditional engagement curve. By removing the extra step of calculating the total on your to-hit roll, the process of rolling becomes the climax of the attack roll, with the hit or miss being the denouement, as opposed to having the process interrupted by adding your to-hit bonus.

That said, addition is the easier than subtraction, and ascending AC is more intuitive than descending AC (because our brains tend to assume that higher numbers are better.) For these reasons, in my ideal version of D&D, AC (and DCs) would ascend with difficulty, the DM would be obligated to tell the player their target number (be it a monster’s AC or a task’s DC), and modifiers would *lower the target number* rather than adding to the result of the d20 roll.

In short, when you described an action, the DM would tell you what number you needed to roll to succeed, you would reduce that number by your modifier (or increase it if you had a “penalty”), and roll to see if you meet or exceed the target.