I believe that people fighting DKs will get a feeling that they are OP. I guess this is because of the novelty of the class. You don't know for sure what to expect from a DK, but as time passes, people will learn about them, and devise strategies, counters, etc. They will be naturally "balanced" (or a nerf will do this, if there's a need for it).

I think there's no overpowered class on WoW.