Oblivion didn't even let you kill certain NPCs! I forgot about that.
If I want to kill the Count of some town, let me do it, damnit! Oblivion is a game that tries to be immersive, but nothing is more un-immersive than seeing some NPC go "unconcious" when I stab them in the back, only to get up, completely unharmed 30 seconds later.
I don't care if the game deems that person important. I don't care if I break the main quest. You let me loose in this massive world, and then prevent me from doing stuff for basically no reason? This is supposed to be an open-ended, non-linear, player-driven experience, not some on-rails story-driven bullcrap.
Skyrim better let me kill, loot, and pillage to my hearts content.