One-hit KOs in a MMORPG. Seriously, anybody who´s ever played one KNOWS that a mechanic like that would not work. It´d ruin the game. And that´s just in game situation. Try imagining it in a life-death situation. Yeah, fuck.
back then, when I played WoW, every boss would 1 hit me.
in GW2 there are several boss abilities which will one hit you too if they managed to hit

it doesn't destroy the game as long as you have mechanics which allow you to bypass the 1-shot kills
dodgerolls, active parry/blocking etc.