Well random npc who give out pretty random quests can be killed in all previous games, npc who is involved in important quest chains can not.

This should also be pretty easy to fix by mods, yes it has some chance of screwing up you game

I'm going a little off topic here, but there are lots of "pen-and-paper" RPG that don't need an all-powerful DM. In those RPG the one narrating the story is in charge to create the world and adapt it based on the players' actions and descriptions. Players' action are reviewed by peer decisions ( i.e. the other players can raise doubts on cheesy moves ). What is more important in those RPG is the result, the fiction, not the story itself.

To return intopic, while I like the approach I described, it is unfeasible (yet) for a software to accomodate it; thus the old approach is still the only valid one.

A game developer has to do what he can to assure that at least the main quest can be completed no matter what. Thus essential characters. Of course the could lose the essential trait once the quest has been completed. I vouch for that.

That's the problem I'm talking about in my comments the important quest NPC's. All quest NPC's should be killable.