Oblivion prevented you from killing certain quest/game critical characters, no matter how much they annoyed you.
In Morrowind you could apparently, if you had the bloodyminded patience, kill every charachter in the world. If the character was necessary to completing the game you were offered a chance to reconsider but the game not stop you. Which is it going to be in Skyrim?
I would like to see the Morrowind version, whilst knowing full well I would fall foul of it by zapping straight past any on screen message. (save and save often)