A max damage crit from a 1st level orc does 24 damage. This is enough to drop all but the toughest 1st level PCs to -10hp instantly. If the orc had a greataxe instead of a falchion, a max damage crit would do 48 damage which would even instakill a 2nd level dwarf barbarian with max Con.
Effectively this is exactly the same as save-or-die but with the rolls to hit, confirm crit and damage replacing the saving throw. In both cases a PC can go from perfectly fine to dead immediately without a chance to do anything about it, like run away. And this is the important part because both crits and save-or-dies make player decision making irrelevant, reducing the impact of skill.
Thus Gary's argument against crits in the 1e DMG applies just as much, in fact more so, to save-or-dies.
My point isn't that you shouldn't have crits and SoDs. I happen to like them as they add drama and unpredictability, which I think is worth more than the loss of player skill. BUT I'm saying it doesn't make sense to have one and not the other. You should have both crits and SoDs or neither.
However immediate actions change all of this because they bring player decision making back into the equation. The 4e Second Wind power restores hit points so it would work against crits but not SoDs. This would be a good reason to have the former but not the latter.