The problem is that a true hero class should be better than all the others. It should be stronger and faster, have better abilities and so forth. Unfortunately, this causes everyone to decry it as Overpowered and thus it gets nerfed. While I love playing my Death Knight, if Blizzard had wanted to keep the game fair and balanced, they should not have introduced DKs or at the very least, not called them Hero Classes. Maybe just Expert Classes or something.