Mitigation and control
We've mentioned mitigation, but even if these were nerfed they still bear mentioning. The Unholy talent Bone Shield is a 20% mitigation that's undispellable because it's a physical buff. Most PvP Death Knights will have this. Then there's the obscenely overpowered -- even after the nerf -- baseline spell Icebound Fortitude. Aside from 20% mitigation again, that's a 12 second stun immunity every minute. It costs a mere 20 runic power, making it easy to have up at all times. Death Knights can theoretically be immune to stuns 20% of the time. Although you won't see it too often, Guile of Gorefiend deep in Frost makes it last 18 seconds, or roughly 30% uptime. It's also a physical buff, making it undispellable. Did I mention it's off the GCD? Yeah.
We've mentioned Lichborne, which is like the PvP trinket on steroids because it not only breaks but provides immunity as the Death Knight becomes Undead. Another talent that makes them Undead is Shadow of Death, which keeps them fighting. Even though no longer usable in Arenas, Unholy Death Knights in the Battlegrounds or in world PvP aren't just hard to kill, you have to kill them twice. If that's not survivability, I don't know what is.
Both Lichborne and Shadow of Death are talents, however, so not Death Knights will be unstoppable, un-CCable Undead machines of doom. On the other hand, Anti-Magic Shell is a baseline ability that makes every single one of them immune to most forms of crowd control for 5 seconds every 45 seconds. Hex, Sheep, Paladin stun, Fear... none of those will work. That's in addition to the 75% damage absorption. It probably won't surprise you if I told you this is off the GCD. Because it is.
The importance of speed
Speed is critical in PvP. This is why Unholy Presence is godly. It's great for DPS, sure, but being able to access important abilities at an instant is so powerful in an environment where everything happens within the blink of an eye. So while most classes lock themselves out of options for 1.5 seconds whenever they cast a spell or perform a strike, Death Knights are only locked out for 1. Furthermore, the good number of mitigation and anti-CC abilities they have off the GCD means that they can freely use special strikes while still having instant access to those spells. Not only do they have access to those spells, but if the situation calls for it, they can activate them all at the same time.
I don't think I need to spell out how stupidly powerful this is. Death Knights have powerful spells off the GCD on relatively short cooldowns. Meanwhile, something as simple as Paladin auras activate the GCD. The Death Knight version, the presences, are off them. Furthermore, Death Knights have access to instant heals. Hybrid classes in DPS specs such as Enhancement Shamans and Retribution Paladins don't have instant heals. Well, Paladins do when they pick up The Art of War. Except that The Art of War procs after a crit, and Flash of Light heals for considerably less than a Death Strike, even when it crits. Furthermore, an instant Flash of Light activates the GCD and resets the swing timer.
We won't even need to talk about the plethora of offensive abilities a Death Knight has access to. I'm sure many of you have tasted that first hand. The reason Death Knights are so powerful in PvP is an accident of design. Many of the mitigation abilities are intended for tanking, but they translate incredibly well for PvP, too. The change to Icebound Fortitude was made so that tanks with high defense could continue to have their own version of Shield Wall on a one minute timer. It's no longer as strong in PvP but you get the idea. The class is designed to tank while dealing good DPS, but the corollary is that the class will be devastating in PvP.
So are Death Knights autowin in PvP? Will the side that has more Death Knights win the match? It's definitely a stretch to proclaim that, but it does have some basis. The class in the hands of a mediocre player can do incredibly well. Now imagine that class in the hands of the best PvP players in the world. That, my friends, is an autowin.