Death Knights have gone through Wrath of the Lich King without a clear idea of exactly what Blizzard intends them to do. Frost, Blood, and Unholy are all viable to varying degrees in both tanking and damaging. Cataclysm will give DKs a clearer path to follow when choosing their role.
Death Knights in Wrath of the Lich King were an extremely versatile class capable of filling many roles. An unfortunate side effect of that versatility was that they lacked a clearly defined role in many situations with each tree being a muddle of defensive, offensive, and utility abilities.
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