I think critical hits were the first thing I house ruled into AD&D back in the day. We felt they added flavor, and there was nothing like rolling a natural 20 and having the critical swing the combat's momentum back in the party's flavor. It didn't hurt that the DM often didn't remember to inflict the criticals on the PCs.
When we moved on to MERP / RoleMaster it was crit heaven. My God but they had the best criticals ever in a game. Some funny shit too. They also led to fairly frequent party member death, as the crits were quite lethal, and the GM never forgot to use them against the PCs.
Tunnels & Trolls 5e and prior didn't have crits. With the way combat is rolled, once the players had momentum going, it was all downhill for the monster side.
That changed with the advent of Spite Damage. Basically all 6's rolled resulted in a point of damage that automatically scored, despite the other side's rolls or armor. Suddenly, solo adventures that were hard were now impossible, as your PC was slowly whittled down (as most T&T solos lacked healing for the player, this was a huge change in game balance).
T&T got me thinking about criticals in RPGs in general. In any particular encounter, the monsters / adversaries are making just that one appearance. For the most part, the GM isn't concerned about any damage they suffer carrying over until later. Not so for the PCs. Whatever damage they take, whatever penalties that's linked to that damage, carries over from encounter to encounter.
Criticals are flash that look great on paper, but are actually a PC penalty.
Is it possible to make criticals fair for the PCs? Are criticals even needed?
Why the hell are we so attached to them? I know I'm guilty of liking criticals, even though I know are far more likely to screw with me than help me.