Class is not irrelevant
That mantra got launched because we did not want raid leaders to have to sit good players in order to build an inflexible list of raid buffs and debuffs necessary to succeed. If you looked at a roster for blade soul gold, nearly every raid looked pretty similar, maybe with 1 or 2 class differences. In Icecrown, the raid comps are enormously different. That does not mean class is irrelevant.