yeah because everyone one loves killing someone important to the main quest, playing for 40 hours, and then realizing you have to lose 40 hours of gameplay or even start a new character becuase you can't complete the main quest
what you want is what I like to call bad game design
however I would like it handled a little more subtle than in oblivion. Let me kill every character but at least give me a heads up if I've broken any major guild quests or the main quests
Removing the invisible barriers and handholding that prevents the player from doing something stupid is hardly what I would call bad game design, if you kill an important quest giver then its your own damn fault, why the hell did you feel the need to kill him anyway? Freedom does not come without consequence, if you want to kill everyone in town the game should let you however dont expect to be able to do this without some severe consequences.
That being said you do know that in Morrowind if you killed an important NPC you would get a message that tells you that you have ruined the main quest dont you?
As for the subject at hand I would like to see the ability to knock people unconcious but not in the way the OP is implying. What I want is stealth takedowns where I can subdue my enemy with a non lethal takedown, would be handy for thief missions where you cant kill anyone, somewhat like this mod.
http://www.tesnexus.com/downloads/file.php?id=9655