In MMO's where health bars are numbers that float up and down, and so long as people get their rotation right, have no risk of hitting zero, one hit kill mechanics are a good way of making you pay attention not just to your job as a DPS/Tank/Healer, but to the mechanics of the fight as well. They give you instant feedback. There was a right way and a wrong way of doing things, and you chose the wrong way. You can't blame the healer or the tank, the blame clearly lies with you for failing to avoid ________. It keeps players awake and fights interesting. Outside MMO's though, combat is more dynamic by nature, without it being split among the three roles of healer, dps, and tank. There are no aggro tables, and generally not characters that can absorb damage forever until the end of time, so you don't need one hit KO mechanics to keep you on your toes.
You just described lazy, half-assed, incompetent design.
Even WoW has changed this by massively inflating the health pools for every class, and also nerfing healing. MMO or not, D2 still stands as the best example as to why it's just awful to have one-shots in a game to make it "difficult".
Instant feedback is not "oh I died within one second". I don't even know what instant feedback is supposed to mean, to be honest, or what it has to do with gaming.
Have you ever played a healer in a game with one-shotting? It's the exact opposite of interesting. Take another type of game, one with regenerating health, and "heals" are based upon increasing regeneration rate, and it is a lot more interesting.