ya in TBC different classes could do different things, druids came to the forefront and could be amazing avoidance tanks (70% dodge in SWP anyone?), pallies were amazing AoE tanks, and warriors were still considered (mostly, but it changed the closer to 3.0 it got) the main boss tank.
in WotLK blizzard focuses more on "bring the player, not the class" which is why all 4 tank classes now are capable of tanking almost anything (anub arak adds come to mind as something druids and DKs simply cannot single tank, but they're the better tanks for anub himself and pallies can also do the adds very effectively, not just warriors). Sure pallies might have a better time than a warrior in an AoE fight, or a DK might do better against sindragosa because of better magic mitigation through AMS, but all the classes are at least CAPABLE of doing the various different roles to the point that it is not a determent to guilds if they only have a DK and a warrior but not a pally or something like that.
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