In addition to other great information here about THACO, it's worth highlighting that it was upside-down.
Lower rolls were better.
This led to magic items giving both pluses and (good) minuses in order to remain consistent in world. Also some curses and ailments gave (bad) pluses to certain rolls.
It was wildly unintuitive.
Indeed.
"With a good fake nose, no one remembered anything but the nose, afterwards."
Yeah, as DM I've always house ruled that it didn't make sense for a character to fail at the thing they're the best at.
Though I have been known to interpret a natural 1 as a crazy external force - like an earthquake - and have them reroll at -10.
Makes it even more fun when they succeed anyway.
Indeed. And when players ask pointed questions to the only NPC who was anywhere near the scene of a crime:
Edit: "Well I heard it wasn't a guy in a hotdog suit. We can't know what to believe."
I like to roll a d4.
4 - Adlib an outcome that is favorable to them, beyond all reason. With my players, sometimes this just means nothing at all happens. In these cases I'll use anything to make it work out for them, from divine favor, to a key NPC breaking rank "I always loved you guys!"
3 - They achieve what they hoped for, and as many weird (but reasonable) side effects as I can think of also happen.
2 - As little happens as is reasonably possible. Often, with my players, that means just 6d6 fire over a 20ft radius. Often after having whatever they tried misfire first, only to have them try again.
1 - I unpack a nice handful of d12 and roll for blast radius, save DC and damage.
Modified, of course, for the situation.
Specific damage on a 2 or 1 should - like anything they couldn't reasonably prepare for - be attention grabbing, but unlikely to be lethal. A 2 should as anticlimactic as reasonably possible.