There's usually only a handful of shows that make the tough decision to kill off an important character. On Grey's Anatomy's sixth-season finale, the show bid adieu to T.R. Knight when George O'Malley died after being hit by a bus. His face was so swollen and cut up, however, that it was only in the final seconds of the episode that George's colleagues realized that he was their "John Doe."
Other examples: Charlie on Lost's Season 3 finale, Kate Todd being shot and killed on NCIS.