Karazhan is unique because 1. the raid was about the story, not going around killing mobs for better gear. It was highly scripted and there were no wings, the progression was linear, emphasizing on the dark plot of the mage tower and it's previous owner. 2. Karazhan is by far the largest raid ever, consisting of cellars, secret room, haunted laboratories, ominous opera theaters and so on. Again, it is not large because there are a lot of gear drop p ing mobs to kill but because of the story. 3. Karazhan was super hard, giving something players to chew.on for a long while.