PVP spec trends
Believe it or not, there are still relatively valid arguments to made for both unholy and frost as PVP trees this expansion. Blood spec, of course, has high survivability but generally is not considered to have a large enough offensive toolbox to be truly effective at high-end PVP, beyond being horribly hard to kill, so we'll focus on frost and unholy builds here today.
If you want to go frost, try something like this linked build and glyph outlay. This build allows for some high-powered crowd control (especially in high-density battleground or world PVP encounters) via Chilblains and glyphed Howling Blast. You'll focus on mastery in your gearing and reforging for this build, with an aim of making your Howling Blast damage as high as possible. While there's no real way to make a rotation in PVP, you'll focus on Howling Blast, using Necrotic Strike for your unholy runes and saving Obliterate for Killing Machine procs.
We're using two-handed weapons for this build in large part because it frees up a couple of talent points for useful stuff like Hand of Doom and Unholy Command but also because the extra runic power means more Frost Strikes.
If you want to go unholy, try this linked build and glyph outlay. Unholy builds have less utility and crowd control than frost but allow for more survivability and slightly more power against healers, thanks to more chances to use Necrotic Strike. You'll focus primarily on haste in the unholy build, the better to keep rune regeneration to its highest levels. For a "rotation", you'll focus on using Festering Strike to create death runes and keep diseases refreshed, then Necrotic Strike to keep targets slowed and unable to be healed.
You'll want to use Unholy Presence for the movement speed, extra haste, and rune regeneration when PVPing in either spec, although in extreme burst situations (especially for frost), it may be worth it to switch to Frost Presence. Watch out for lost runic power, though.
Choosing which spec is right for you is as much a matter of personal preference as anything. Do you do well with one spec in PVE? Maybe you'll want to try it out in PVP. Do you want to control the battlefield? Frost may be more your bag. Also, pay attention to your spec's special features. If you have lots of physical-damage-causing teammates, Brittle Bones may make frost worth taking. If you're teamed up with casters, they may appreciate the power of Ebon Plaguebringer. Of course, don't feel shackled by that, either; other members of your team may be able to provide those debuffs instead.
Gearing up to get down to business
For the most part, gearing for PVP is a little bit easier than gearing for PVE. There's one basic set; you buy it, and when a new set comes out, you buy that instead. There are still a few things to keep in mind, though.
First, you'll want to make sure you have 5% hit. This will allow your special attacks and two-handed white damage to hit level 85 opponents every time. Second, you'll want to make sure you have around 195 spell penetration. This will allow you to bypass the resistances afforded by player cast buffs, totems, and auras.
Once you have those two caps met, you can focus on resilience. Never underestimate resilience. Without it, you're probably going to die and get frustrated. With it, you'll be a wrecking ball.
As far as secondary stats go, you won't have too many choices there, since most slots will only have one option for any given gear level. Still, you can reforge your nonresilience stat where needed, and sometimes you will get a choice between multiple options in certain slots. For frost PVP, you definitely want to focus on grabbing mastery where you can, since that will contribute to harder-hitting Howling Blasts and Frost Strikes. For unholy, you can stick to haste, since you'll be using Necrotic Strike just as often as you use Scourge Strike, if not more, making mastery less desirable.
If you're looking to break into PVP to start, there are two relatively easy ways to get your gear. Blacksmiths can create resilience gear, so if you have a lot of gold to spare and/or a blacksmith character of your own, you can simply spend gold and get a nice basic set of resilience gear that you can use to jump
If you don't want to spend the gold, it may be a better idea to spend justice points instead. Your faction's trade goods JP vendor should also have an option to exchange 375 JPs for 250 honor. You can then use that honor to buy a few pieces of resilience gear. As of this writing, there is some nice ilevel 371 gear you can buy with honor that carries zero requirements as far as PVP ratings go.
Shutting them down and tripping them up
As a death knight, once of your biggest jobs is going to be locking down casters. You'll want to learn how to use such abilities as Strangulate, Mind Freeze and, when needed, Death Grip to keep casters from casting. You'll want to learn to use Anti-Magic Shell to protect yourself and Necrotic Strike to keep spells easier to interrupt and to keep healers from nullifying all your damage. This is job one for most death knights, so learn it and love it.
You'll also want to learn how to keep people immobilized as much as possible. As death knights, we aren't blessed with gap closers like Shadowstep and Charge, so we'll have some trouble with this. Luckily, we do get Death Grip, but don't forget Chains of Ice as well. This is also where speccing frost for Chilblains comes in.
The best way to use these skills, though, is to know your opponents. Your opponents have ways to get free of you, whether it's a rogue's Sprint or a mage's Blink. It takes some concentration, but if you can anticipate when they'll use these type of cooldowns, you can save cooldowns like Death Grip and reel them in while they're helpless to escape. Unholy death knights will also want to keep Gnaw off auto cast so they can use it for immobilizing as well. It's a lot harder to hit a moving target, so the slower you can keep your prey, the better. It's also worth it to note that sometimes, it's not just about keeping your prey immobile -- it's also about keeping them away from your partners. If your healer or your favorite caster is under attack, Death Grip and Chains of Ice can get him out of trouble.
Practice. Queue early, queue often. Look for unorthodox ways to use your skills. Learn to adapt to the ever-changing nature of the battlefield. Don't be afraid to fail. Communicate with your teammates, but don't whine. These are the basics, so you still have a long way to go 'til mastery, but keeping the basics in mind at all time is still the mark of a true master. We'll see you on the battlefield.
Learn the ropes of endgame play with WoW Insider's DK 101 guide. Make yourself invaluable to your raid group with Mind Freeze and other interrupts, gear up with pre-heroic DPS gear or pre-heroic tank gear, and plot your path to tier 11/valor point DPS gear.