Breakfast Topic: What characterizes WoW's best raids?

People disagree a lot as to what WoW's best raids have been, not least because it's tough to evaluate how "good" a raid was until after some time has passed. For example, an MMO Champion forum poll a few months ago selected Icecrown Citadel as the game's best "final raid," and yet, I remember lots of players complaining that it hadn't been anywhere near as good as Black Temple a the time. You also have to wonder about the extent to which accessibility plays a role. Most people who saw Sunwell at level 70 loved it, but that wasn't a lot of people.

Anyway, three of the most frequently cited "best raids" of WoW are Karazhan, Black Temple, and Ulduar. I thought it might be worthwhile to ask -- what made them so good?

• Iconic drops or rewards Many of the drops from these places are instantly recognizable. Everybody wanted the "squid staff" off Illhoof and the Nightbane shield, and you can't swing a dead cat in trade chat without hitting another Black Temple transmog run. Even Ulduar, in the age of gear consolidation, had memorable drops.
• Atmosphere Karazhan was essentially an overgrown haunted house. Black Temple was a fortress full of Broken that would be hostile until you set Akama's soul free, with the brooding Illidan perched on top. Ulduar was an exceptionally beautiful fortress concealing its true purpose as a prison.
• At least one unexpected encounter You couldn't beat Kara without besting Medivh's ghost at chess, Illidan was one of WoW's most inventive fights, and, as nightmarishly difficult as it was, I still cherish my guild's first Yogg-0 kill.

What made something WoW's "best raid" for you? What mechanics or features would you most like to see repeated in a future raid?

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