Critical hits. I played 1e and 2e without them except for briefly when critical hits and misses were tried and no one liked them so it was quickly dropped. I played mostly 2e through the 3e and 4e days, though I played some 3e too but only have vague memories of confirmation rolls. Now I wonder how many crits in 5e my group has simply forgotten to roll the extra damage on a natural 20. I mean, the players must be rolling 20s now and then, and I must be rolling them for the monsters, but I don't think we have ever rolled extra damage. I think it would only matter for a champion fighter anyway, which no one is playing at the moment.
It would be cool if crits meant something more than Moar Damage. If instead of damage it could shove an opponent prone or away I think it would be more interesting and hence easier to remember.
Edit: I just remembered the time in during the Next playtest where I TPKed everyone because some orcs got 3 crits against the party. I think it cemented the opposition to crits because of the swingyness it adds to combat. Maybe everyone is happy to forget crits on purpose? Or they culd just be as forgetful as I am.
