When raiding started in vanilla World of Warcraft, there was one choice for a main tank, a protection spec warrior. The off tanks were dps speced warriors and that was it. Occasionally you had a feral druid off tank, and when dealing with undead a paladin would sporadically step in and help out. This was the height of warrior dominance in tanking; there really wasn’t any other choice. If your main tank didn’t show the raid would be canceled and 39 other people had nothing to do for the night.
Then came the Burning Crusade expansion and everything changed. Suddenly druids and paladins became capable of tanking instances. Druids were the first main tanks in Karazhan. They didn’t need much gear to be defense capped and had large health pools. Soon warriors returned to their dominance once they got gear, they kept the main tank spot thanks to tradition and some gimmick fights that required spell reflect on bosses.
But then something changed near the end of the expansion. Paladins gained enough stamina to catch up on the gap between them and warriors. Also everyone got tired of using CC on mobs, thanks to consecration, paladins became the tank for 5 mans. This was seen the most once Magister’s Terrace was released, most dps refused to run the instance unless a paladin was tanking.
Now with the release of Wrath of the Lich King there is another tanking class in the mix, Death Knights. Much like druids and paladins before them, they have some ground to catch up on. Guilds are using tanks that aren’t warriors on new content and being just as successful. Warriors are still seen as THE main tank. But this is not the case; it is tradition that holds warriors on the top of the heap. Every other class can hold their own at the same gear level a warrior can with the same results. There are spread sheets that prove every tanking class as the best. And with less gimmick fights in this expansion that require a certain kind of tank, every class is proving the sheets true. We have moved past the time that warriors are the have all be all tank and really the concept of main tank may even be going by the way side. Knowing every tank’s strength and weakness in situations and checking your ego at the instance portal can help immensely when it comes to tanking and progress. And that is exactly how it should be where the best player gets the main tank spot in a raid, not a certain class.