Handling character death in D&D

Character death is a tricky issue in any roleplaying game: On the one side players can feel an immense sense of loss if they really lose a character they got attached to; on the other side having death become too trivial leads to all danger being perceived as trivial as well. In a pen & paper roleplaying game the likelihood of a player death will vary a lot between different groups. It is up to the Dungeon Master of a D&D game to get the balance right. How do you make death significant,

World of Warkraft...