Ah.. THAC0... I've missed you. But, alas, you live on in 5E with the "number needed to hit the AC".
I still calculate the number I need to roll to hit an AC. For example, the Fire Giant is AC 18, and the PC has +11, so if they roll a 7 or higher they hit.
Traditional d20 Player: AC 18? Ok.
I rolled a 7...+11... is 18. So, I hit.
I rolled a 12... +11... is 23. So, I hit.
I rolled a 3... +11... is 14. So, I miss.
THAC0-based Player: AC 18? So, I need a 7.
I rolled a 7. Hit.
I rolled a 12. Hit.
I rolled a 3. Miss!
It is easier to do the math once than every time you roll the d20.
(Emphasis Mine)
This. You did the match ahead of time to generate your "to-hit" chart; of course, once you started swapping among magical weapons with different +X values, you were probably "adding the plus to the roll" dynamically rather than "shifting X columns over on the to-hit table" in most practical settings.
As was mentioned upthread, THAC0 wasn't as counterintuitive as people made it out to be; the issue is that the Armor Class system was a holdover from Wargaming and its shorthand nomenclature was counter-intuitive. Why does a descending armor class mean improved armor? Use cardinal numbers, not ordinal numbers. To wit,
Would you rather have "first-class armor" (AC 1) or "second-class armor" (AC 2)?
I think once you phrase it in that way, it makes intuitive sense that "first-class armor" is better than "second-class" armor (or "first-rate"/"second-rate" if you prefer), so saying "lower AC is better" makes intuitive sense once you've seen that phrasing of it.
The problems come into play when you introduce magic plusses into the equation and allow armor class to go below 1 (what is "negative fourth-class armor"?) It leads to the silly prospect of a fighter wearing plate mail and a shield (AC 2) picking up a +3 shield instead of his nonmagical shield and having a resultant armor class of -1 (yup, 2 + 3 = -1).
Also not brought up in this thread, some early editions of D&D had "ability checks" (or proficiency checks if you used Non-Weapon Proficiencies) where you rolled a d20 with some modifiers and tried to roll UNDER your relevant ability score (which is why a 3-18 spread for abilities was useful instead of today where for all intents and purposes abilities are
really -4 to +4).
The perceived problem with older versions of D&D is that different situations used different rules (Saving Throws? Roll high on a d20 against a flat table against an arbitrary save type. To-Hit? Roll high against a table where you have to cross-reference Armor Class. Non-weapon proficiency? Roll low on a d20. Thief skill? Roll percentile dice and consult a table... and roll below the target number to succeed. Searching for secret doors? Roll a d6? Rolling for initiative? Roll low on a d10. Magic Resistance? Roll high on your percentile dice or the spell fails.)
The "great innovation" of third edition was in fact the d20 mechanic - the consolidation of all of the various systems and their diverse types of rolls into the same mechanic" "roll a d20, apply modifiers, check if the result exceeds a target number, higher is better"). Interestingly, there were deviations from this mechanic even in 3e ("critical threat ranges" and "rolling to confirm a critical" meant combat REALLY wasn't "roll a d20" it was "roll 2d20 and sometimes ignore - or don't roll - the second d20"). We've seen deviations introduced in 5e (spells and abilities like Bless that have you roll a d4 and add it to your d20 hit roll, for example) that get farther afield from this "unified" mechanic. Instead of Magic Resistance we have Legendary Resistance. I could go on.
In other words, we've just re-introduced "small variation" elements from previous editions of D&D that brought interesting tactical options over the years. What are some of the things we've lost?
* Weapon speed factors and spell casting times (1e/2e) that adjust your initiative roll so casting a high-level spell or wielding a huge weapon means you're likely to lose initiative to someone casting a low level spell or wielding a small weapon (back when losing initiative as a caster often meant taking damage and losing your spell).
* BECMI's "Weapon Mastery" system where different weapons provided significantly different tactical options (deflect, delay, disarm, etc.) and damages based on your skill with the weapon.
* 3e's use of "critical threat range" and "critical multipliers" and "double weapons" (yes, I know, everyone mocked the double-bladed sword for being Darth Maul) for various weapons made weapons more varied and interesting (an axe does x3 on a crit but only has a chance to crit on a natural 20 while a longsword does x2 on a crit but has a chance to crit on a 19 or a 20)... at least until the spreadsheet folks among us optimized the system to death.
Of course now 5e24 and 5e derivatives are spicing things back up with Level Up's Combat Maneuvers, Tales of the Valiant Weapon Mastery, and countless third party sourcebooks trying to give different families of weapons different mechanical feels in combat.
To wit, while 3e/d20 tried to consolidate everything into "one mechanic to rule them all" the proliferation of variations off the "roll a d20, apply modifiers, compare to target number, roll high" means that perhaps gamers DON'T want everything simplified. I think the real improvement of "adding numbers to a die roll" is something most people can do in their heads and means we don't have to carry around tables with us to consult (which THAC0 generally needed, at least for your pre-modifier number).
But I would contend "consult the table during character creation to find your THAC0" isn't
that much different form "consult the PHB during character creation to find the rules for your spells/feats/skills/what have you." I'll bet most of us still have to consult tables to correctly know how many spell slots of each level our casters have (maybe not at first level, but your base THAC0 at first level was always 19 in 1e so that was easy)... and we don't poo-poo that table lookup and tell casters they need a better system (well, I suppose those that want a spell points system do, but you get the idea).
