One could make the "Bloodied" condition and "total surprise" situations nastier by allowing save or die effects then. (Or maybe just "die" effects, actually?) If you're so unprepared, or so weakened, you can die by a single spell or a knife attack to the head. But if you're prepared and not too weakened, the enemies spell won't really hit you, and that knife might be aimed for your head, but won't hit it.
Generally, I think it may be good if the system allows "engineering" situations where you can kill someone instantly. Ambushes, whittling down someone's hit points so he's weakened enough. but it shouldn't be easy, and ultimately, it should require some level of work on the attacking side and some level of failure on the defending side (e.g. not forseeing the attack).
Theoretically, 4E D&D allows a single Intimidate check to take a bloodied NPC out of a fight by forcing him to retreat. It seems a mere technicality to also have some kind of arcane deaths spell that does something similar, except the enemy dies from it and doesn't merely retreat.
Also to keep in mind:
A monster like the Medusa in mythology is a special monster. There are no Medusa Archers, Medusa Spellslingers, Medusa Minions (4e speak), nor Medusa Fighters and Medusa Barbarians and Medusa Rogue/Assassin (3e speak). There is this one, unique monster that petrifies you if you encounter it. You won't encounter it by accident, because people fear it and know where it is, usually.
Such a special "story creature" can be allowed to do things that monsters that you meet by the dozen in random encounters. Just as the story as you use it will also offer some counters if the party is smart enough to figure them out.
If you put save or die abilities on spells, then such spells should be equally rare. A 15th level Wizard shouldn't slot 20 spells from which 6 can be save or die spells. Maybe he should be able to slot only 5 spells, and one of these is reputed to be a deadly spell, allowing him to kill a person just by pointing his finger at it.