WoW tried the magic tank thing with DKs.

Didn't work well. With five classes that can tank(monk, druid, Paladin, death knight, warrior as soon demon hunter) they can't really get away with "best tank" for long because that's way too big a balance issue.

Kinda why they're all copycats of each other. Druid/monk are dodge tanks. Paladin/warrior are shield block tanks. Death knight is a halfass heal/absorb/parry tank.