I think you've hit this one right on the money.The classes that everyone always complains about being OP are the classes that are fun to design. Locks, Rogues, and now DK's offer lots of neat ways to hurt people, which is always the most fun part of a game where the ultimate goal of basically anything you do is to kill something. Being able to turn into a giant demon? Sucking the life out of everything around you? Being able to completely control someone and laugh as you kill them without them ever even touching you? These are all FUN things to do in game. Compare that to putting a glowing shield on someone or fixing their booboos, or throwing conjured balls of fire at someone. It's just not fun to design. That is where the balance team steps in and says "Wow, I bet that little move you just did is a lot of fun to do. Have you tried it out on the receiving end yet?" and things get fixed. Prime example is Atrocity. Warlocks were going to get an AoE Corruption. Every Warlock started drooling, especially when that was combined with Haunt, which hits targets that have Corruption on them. That kind of thing is fun to implement, and I'm sure very fun to play. it also makes it pointless to play anything BUT a warlock. It was removed shortly into the Alpha when people started really looking at its potential.

The DK trees are the same. You have a total badass class that feels like a total badass. Everyone that plays it wants to feel like a badass. But when everyone plays a badass, the game suffers, and these trees are in the nerfing stage because they are simply so much better than everything else right now. The only thing that comes close are Ret pallies, and they are about to get hit with a pretty big nerf bat too.