The unholy tree is widely considered to be the most effective for PvP, so players should expect to see it a lot on the battlefield. As mentioned, the most telling sign of an unholy-specced death knight is the persistent ghoul with a unique name. Better death knights will be able to micromanage their ghouls such that they can Gnaw at strategic moments, effectively having another interrupt in their already considerable anti-caster arsenal. Most death knights, however, will simply have it on auto-cast.
More than any other spec, the Unholy death knight relies on stacking diseases upon diseases, so classes able to remove them should do so at every possible opportunity... even their auto-attacks benefit from diseases with Blood-Caked Blade. Naturally, unholy death knights also inflict more kinds of diseases than other death knights and capitalize on this with their bread and butter attack, Scourge Strike.
Another wonderful PvP talent is Desecration, which ensnares opponents in a wide area. In conjunction with their persistent increased movement speed regardless of Presence and a reduced cooldown on Death Grip, unholy death knights are particularly difficult to peel from. They can also sic their ghouls with a Leap and a Gnaw in order to catch up with opponents.
Unholy death knights are basically a pet class similar to hunters and warlocks. This means it can sometimes be worth the hassle to kill the pet, although you should expect the death knight to Raise Dead almost immediately, necessitating killing the pet twice. Alternately, some classes can keep the ghoul crowd controlled with a Shackle Undead or Turn Evil. Keeping the ghoul out of the action will help against an unholy death knight.
Death knights can also summon a gargoyle, which deals respectable DPS and can cause a lot of harm if left unattended. This uses up a lot of Runic Power, so expect it when the death knight needs a Runic Power dump. Gargoyles can be targeted and destroyed or, just as with the ghoul, kept crowd controlled. Keeping it crowd controlled is best because it discourages the death knight from simply sacrificing it to a Death Pact. Some death knights will cast Death Pact on a ghoul or gargoyle being attacked in order to recoup some health rather than losing the pet to opponents.
Unholy also has Bone Shield which the death knight will throw up whenever it's off cooldown in PvP, making them tougher to crack. Bone Shield is a physical buff and cannot be dispelled, so the best way to remove it is to attack fast in order to consume the charges quickly. Bone Shield is on a one-minute timer, so removing it as fast as possible after it goes up is key to making the death knight more susceptible.
The bad news is that death knights are everywhere. They're a very popular class and encountering them is unavoidable. The good news is that most death knight players suck. With due apologies to the death knight player population, this is a direct result of the class being relatively new and very few players will have played death knights long enough to understand all the nuances of the class. There also isn't enough knowledge about death knights compared to other classes, so resources are few and far between (Daniel's awesome Lichborne column is one of the oldest death knight guides available).
Many death knights, for example, open with Death Grip, even against a hunter. Many will use Icebound Fortitude prematurely. A lot of these small sub-optimal plays are a result of players not knowing their class thoroughly and also because the class has also changed quite a bit throughout its short history. In PvP, take advantage of this lack of familiarity. The death knight has been nerfed considerably such that most players with reasonable skill can take down a death knight. It used to be harder then when the class was ridiculously imbalanced.
Other reminders... avoid fighting on top of Death and Decay. Many death knights will pop Death and Decay at the beginning of a fight which should be taken as an opportunity to keep off the affected area. More experienced death knights will pop it in the middle of an encounter, when snares have been applied or Desecration is in effect, to maximize (and obscure) the effects of the AoE.
Because they are a melee class, disarming their weapon works well against them as many of their special attacks require a melee weapon to execute. They aren't without other tools, however, as Death Coil or Howling Blast, for example, can be fired off without a weapon. Death knights also do not enchant their weapon. Their weapons are runeforged, so there should be no fear from any wacky enchants. Some players will PvP with the Rune of Swordbreaking or Swordshattering specifically to counter disarm mechanics. Others will use Rune of the Fallen Crusader to keep their focus on dealing damage and survivability.
It's always important to pay attention to the death knight's Runic Power. Runic Power management is an advanced death knight skill that not all players can maximize. Players who keep dumping their Runic Power often become susceptible because they won't have access to particular abilities that might be clutch in certain situations. A few death knights keep their Runic Power almost full and use it only to prevent wastage from Runic Power gains, always leaving enough to allow them to pull off important strategic moves. Here's a handy list of death knight abilities that require Runic Power for your reference:
Maximum Runic Power: 100 (130 with Runic Power Mastery talent)
20 Runic Power
While knowing these abilities won't prevent the death knight from using key strikes that use runes, it should give players a better understanding of the two important resources that death knights use. Again, I will stress that these introductory guides aren't a comprehensive guide to fighting death knights. Many players will have key insights on how to deal with death knights and particular death knight specs. I encourage all readers to share their experiences and knowledge in the comments section. Last week's article had some great input and I fully expect this week's installment to be just as enlightening. Next week, we'll take a look at how to handle hunters! That should be interesting.