I got down voted for saying this elsewhere, but to my mind there's a huge difference between the GM unilaterally changing the rules, and the group deciding.
Scenario: the goblin rolls a crit that'll kill the wizard. This is the first scene of the night.
Option A: GM decides in secret that's no good and says it's a regular hit.
Option B: GM says "I think it wouldn't be fun for the wizard to just die now. How about he's knocked out instead?". The players can then decide if they want that or would prefer the death.
Some people might legitimately prefer A, but I don't really want the GM to just decide stuff like that. I also make decisions based on the rules, and if they just change based on the GM's whims that's really frustrating and disorienting.
There's also option C where this kind of thing is baked into the rules. And/or deciding in session 0 what rules you're going to change.