I would just like to add one little thing. I think the biggest outcry of being OP is coming from warriors. Death knights are like warriors but without so many weaknesses. Lemme make a quick list of comparisons.
1: Stances vs Presences. Right now Warriors have to switch stances to use certain abilities, at no time do we ever have access to everything. We also suffer major penalties for being in different stances, AKA -10% damage done in defensive stance, and +10% damage taken in Berserk stance. Death knights suffer nothing from being in different presences, and no matter witch one they are in, they have access to every ability. Switching to frost presence is the same as a warrior putting a shield on and switching to defensive stance, but without suffering for it. Suffering from a loss of abilities needed in order to be viable in PvP, a loss of straight up damage, or survivability.
2: Strikes. Warriors get 1 instant cast strike ability with 31 talent points. Death knights get many that have various effects. Mortal strike has a 6 second cool down and can spec into 5 and blood thirst has a 5 second cool down. Death knights can hit you with at least 2 obliterates, and 2 heart strikes before all of there runes are used up. that's a whopping 4 strikes to a warriors 1. 2 if you count whirl wind. Everything else a warrior has is off procs.
3: Spell damage. Death knights get dots, shadow DD, Shadow AoE, Frost DD, and a lot of it is just part of the normal DPS rotation for them. Warriors are pure physical damage, so it is difficult for a warrior to beat a death knight when a death knight can bypass most of a warrior's armor.
4: Range. Warriors have piratically no range. Death knights have a lot of range. They have a range snare, a range pull, a range silence, and range DPS. They are deadly up close, but they also have a lot more versatile tools than warriors do.
5: Mitigation. Warriors have high armor and health, but nothing to mitigate spell damage. All Warrior Mitigation is only really usable in defensive stance and with a shield on and most of it only works on certain things. Death knights have several tools to soften blows, and usually keep them up. Making death knights extremely tough and difficult to bring down, even for casters. The downside to this is while warriors have to give up a lot of damage in order to be tougher, Death knights do not. They are able to just be tough whenever the cool downs are up.
6: Army of the dead. This power just destroys everyone in PvP. Seriously what does anyone have that even comes close to competing with this in PvP? Holy wrath? Divine storm? Lay on hands? these come to mind but they do not seem as overpowering as this one power that all death knights get. Maybe if a warrior uses shield wall he might be able to live threw this onslaught for 12 seconds, before getting destroyed. Honestly if anyone can think of a counter for this ability lemme know. It might be a joke for a mage, but that seems unlikely since a death knight will just death grip you back into the ghoul's range and chains of ice you.
Basically the point of this post is, Warriors used to be the most feared class in PvP, and now a death knight is very clearly better. I've killed a few Death knights it's still possible, but any death knight right now in frost presence just kills me. Death knights it feels like have no glaring weakness right now. Nothing that can really be done to defeat them. It's less work for a stronger class, and most of the useful PvP powers for Death knights they just get standard. Warriors have to spec into most of the tools they need for good PvP.
Have they killed WoW? No. Are they OP? Yes. Does it make me mad? Not really, I've been a warrior for a really long time. We've been kicking ass and taking names longer than most other classes can brag. It does feel unfair that they don't have to work as hard to get better skills than a warrior does, and it certainly is different to have a class out there that simply can beat me hands down if the other person isn't a complete retard. But I don't let it bother me much. Death knights I feel do need a serious overhaul to be a bit more fair, but I'm not expecting it to come. I have a feeling Death knights are going to be the new kings of PvP, and all the PvP nuts out there will roll Death knights.
I will say with Death knights in the game now I feel really really old. Death knights were built from the ground up, based on a lot of the best stuff from a lot of other classes. That is part of why it feels like they have no weakness. My warrior feels barbaric and stone aged compared to all the death knight strikes dots and coils.