Death Knights. Critical QQ wrote an interesting post in his blog theorizing that the side with the most number of Death Knights will win. Think Ret Paladin during the first week or so of Patch 3.0. He posits that Death Knights are such a powerful PvP class that they tend to turn the tide of a battle. To a degree, it's actually true. The very first Arena Master was a Death Knight.
The blog post plays out several scenarios where Death Knights make a difference but doesn't get into too much detail. But the point he makes is clear, and some of you might have actually felt it whenever you played some Battlegrounds -- whichever side has the most Death Knights wins. I've never actually stopped to figure out the ratio of Death Knights in all my Battlegrounds games, but there's no question a Death Knight is a force to be reckoned with in PvP. But are they unbeatable?
The obvious answer, of course, is no. But they sure are damn hard to put down. Blizzard was aware of this and severely nerfed Unholy in Patch 3.0.8, cutting down the duration of Anti-Magic Zone, lowering the mitigation of Bone Shield and Icebound Fortitude, and putting an unnecessarily long cooldown on Shadow of Death as well as removed it from Arena play. The last change was needed, but the 15 minute cooldown is probably too long considering Blizzard also nerfed the duration to 25 seconds. They also toned down the class' burst damage somewhat, including the amazing Gargoyle talent.
The non-healer healing class
So how come Death Knights are still extremely powerful in PvP despite the mitigation nerfs? Have you ever fought a Death Knight and wondered to yourself why your health keeps getting low while theirs is getting high even after you've hit them with everything you've got? Something that's been overlooked for the most part in the spate of recent balance changes is the sheer awesomeness of Death Knight self-healing. Mitigation on separate cooldowns and a variety of self-heals make Death Knights one of the most resilient classes in the game.
As a baseline ability, Death Knights have Death Strike, a melee strike that heals them as long as they have stacks of diseases on their target. Once Death Knights have set up, it becomes very difficult to stop them because they'll wear you down while they pump themselves up. Then there's Death Pact, the legacy Warcraft III ability, that sacrifices an undead minion such as a ghoul or gargoyle in exchange for health. Note that Death Pact works off maximum health and not current health, which means it's an excellent ability to use when their pet is about to die.
Blood has access to Rune Tap, as well, which when improved can heal them for 20% of their health every 30 seconds. Prior to the nerf, the most popular PvP builds were a mix of Blood and Unholy which had access to frightening healing as well as mitigation. Blood / Unholy Death Knights will use either Scourge Strike or Death Strike depending on whether they need healing. Because both strikes use Unholy and Frost runes, this leaves a Blood rune or two for Rune Tap at a moment's notice. That's not even the best part. Rune Tap is off the GCD. Insane? Sure it is.
While not a popular build for PvP, some pick up talents like Bloodworms, Blood Aura, and Vampiric Blood, which add to self-healing. Vendetta is awesome for a continous grind -- not just for mobs, but in Battlegrounds PvP or Wintergrasp. Sure they'll need to land the killing blow to proc it, but they're Death Knights. Killing blows are second nature to them.
Now, although Unholy is still the PvP tree, it's now more common to complement it with Frost all the way up to Killing Machine. Players speccing Frost / Unholy pick up Lichborne along the way, which is an incredible anti-CC ability on a three minute cooldown. Aside from breaking Charm, Fear, and Sleep effects, it provides immunity to it for 15 seconds. Guess what? Just like Rune Tap, Lichborne is off the GCD, as well. Here's another Death Knight trick -- when under the effects of Lichborne, they can heal themselves with Death Coil. So you can add that to the list of effective self-heals.
Understand that these heals can tick for a lot. Death Strike can crit, which sometimes results in heals upwards of 5,000 hp. This means that while they're dealing damage, they're healing themselves at the same time. Because Unholy Presence cuts the GCD by .5 seconds, Death Knights can blow two Death Strikes in quick succession to heal themselves for a considerable amount. If they have a lot of diseases stacked, that's a lot of healing. Ten seconds later, they'll be able to do it again.
The best (or worst, if you're up against a Death Knight) part is, all of these heals are instant. None of them have a cast time. Rune Tap is an instant heal off the GCD on a thirty second cooldown. Death Strike is a heal that is part of an attack, usable twice in succession every ten seconds. Death Pact is instant on a three minute timer. Bloodworms just happen. All Death Knights will have access to Death Strike and Death Pact, while the others are talents. Instant, uninterruptible self-healing on one of the most devastating DPS classes is equal to PvP godhood.