In D&D, once you get past level 3 or so, it gets really hard to be killed with a single hit, beyond level 5 or so (and certainly beyond level 10) it's pretty much impossible. Characters reach a point where the number of hit points to take them down just can't really be dealt by an opponent in a single blow.
Some people like this, some people don't and there are games written to specifically avoid this concept. I guess it all depends what you want a game to be about...if you want consistency in story and don't want random whim taking out your characters, it makes sense to have a system in place where characters can't die suddenly, without a moment's notice. If you want dangerous and gritty, then you probably don't want a system where characters can take full strength hits from a two-handed sword and just walk away.
I like the way the new edition of Warhammer 40k has gone back to the idea where anyone could theoretically take down anyone else, a comparison of attacker's strength versus defender's toughness gives a simple die roll target number. Some situations moght see instant kills quite likely, others might see it quite difficult to achieve, but as lomg as there's a hit, then a death blow is possible.
I'm seeing a similar system develop here... attacker rolls to hit (which incorporates Defender's defence score), if the hit strikes, any damage is rolled based on a comparison of strength vs toughness,
I'm not sure where I'm going with this... it's just a kernel of an idea. But I want to play with it at some stage.