I can't think of one essential NPC that I had a burning need to kill and couldn't.
I kinda liked the suggestion that NPCs be immortal to other NPCs but not the PC, but the problem there is this: Let's say in the event of a dragon attack, any given single person (maybe a merchant like Lami or minor quest-giver like Roggi) will survive a dragon's attack, probably fighting back until, impossibly enough, they have slain the dragon. It kind of takes the imperative off the player, you see? There's no need for a hero if any given NPC in the game is immortal and can slay the most fearsome of foes.
Im not trying to kill anyone.
I in fact go out of my way to ensure everyone survives.
But that is not the point. The point is that it breaks immersion to have half a world unkillable.
In Morrowind I could kill Vivec, if I was powerful enough. That is how it should be. I didnt, but the fact that I could made it a better RPG.
When NPC's get attacked by a random dragon, they shouldnt just keep on fighting.
They shouldnt die either. So have em 'fall to a knee' and remain like that for the duration of the fight.
New Vegas was excellent in that everyone really important either had a replacement or was in a place not easily gotten to and well defended.
Essential NPC's should have no place in a TES game.
If I break a quest, oh yes please let me be able to do that.
Im sick of games so simple a 5 year old can play them, because failure is impossible.