Death Knights: everyone can roll one at 55 now, and even though the abilities are parceled out pretty slowly as you level, it can still be overwhelming to pile on all those abilities so fast as you rocket through Outlands. Plus, you have all those talents that you don't know what they do. Why not put all 61 points in the blood tree?! Plus, their runic resource system is totally different from that time-tested blue bar you're used to.
It can get pretty confusing. Some abilities cost one rune, but some cost 2 or three of different colors, and some cost runic power, but it's not clear how you get runic power, and some cost a kitten and sixpence, which is way more than a bushel.
By this point, many of you have tried a DK, but most lay abandoned in Hellfire Peninsula after you experienced that completely epic starting area and haphazardly spent all those free talent points in whatever looked cool. Now he or she is sitting in the Thrallmar Inn with a free talent reset and you have some free time between Ulduar raids, so it's time to pick that baby back up!
Hopefully in the future I'll find the time to wax poetic about what a design triumph DKs are as a class. But for now, I'm going to try to boil DK mechanics into the simplest form I possibly can.
First, the basics of how Runes and Runic Power (RP) work.
You have 6 runes: 2 Blood (red), 2 Frost (blue), and 2 Unholy (green). You cannot change this, you will always have two of each. Going forward, I will use B, F, and U as shorthand for the runes. Once you use a rune, it goes on a 10-second cooldown, then you can use it again. This keeps you from spamming the same button over and over.
You also have an RP bar (not to be confused with a place to get a drink and meet people named Drzzzztz or Seferothh with Dark Pasts), which normally goes to 100. When you use a rune, you gain RP. If the ability you used takes 1 rune, you gain 10 RP. If it took 2 runes, you gain 15 RP. Death and Decay, which costs 3 runes, gives you 20 RP. Pretty much any ability that doesn't cost RP generates it. There are other ways to get RP through talents, but let's not complicate things with that just yet.
Since using a rune puts it on cooldown, the sooner you use it the sooner you can use it again. So use rune abilities early and often. You should generally wait to "dump" your RP until either a) all of your runes are on cooldown because you were a good little DK and used them all up just as fast as you could! or b) you are about to cap out RP, which would make any further RP generation go to waste.
Now let's build on that by talking about skills. When you start, the game gives you just enough skills to get by: Icy Touch (F), Plague Strike (U), Blood Strike (B), Death Strike (FU), and Death Coil (40 RP)
Already, you can see something interesting. There are no abilities that costs two of the same color rune, and the only two-rune ability costs FU, and no B.
IT and PS each place a different disease on your target: Frost Fever and Blood Plague, respectively. Your Blood Strike and FU moves (like Death Strike) get stronger if your target already has the two diseases on them. Such is life as a DK: in general, you want to open with IT and PS to get your diseases up at the beginning of every fight. Get used to it. Once diseases are up, unload your runes in Strikes, then use Death Coil when all runes are on cooldown. You could seriously just grind to 80 on single-targe mobs playing this way, if talents didn't have to appear and muck everything up.
At base, your other B rune abilities are Pestilence and Blood Boil. They are both only useful in AoE situations, as is Death and Decay. DnD is a strong aoe spell, and Blood Boil is a weak one, but cheap. BB gets stronger if the target is diseased. Luckily, Pestilence will spread diseases around for you. A very strong aoe attack for any spec is to drop DnD, cast IT and PS on a target, then spread the diseases with Pestilence. See how conveniently that uses up all 6 runes? Now you're thinking with portals!
Since your F and U runes are generally tied up in FU attacks or the disease combo, you'll rarely get another ability that uses just one F or U rune, and if so it will be in the talent tree. Pretty much all of your other rune abilities are from talents. You do get the ability to root using Chains of Ice for 1F, but that's mainly used in PvP.
Meanwhile, your other base options to use Runic Power, aside from the direct Death Coil damage (remember, it's different from Warlock's Death Coil because it has no fear effect), are defensive. If you have some RP, you can use Icebound Fortitude for general defense, an Anti-Magic Shell for magic (duh) defense, or use Brain Freeze (an interrupt like Kick or Counterspell) or Strangulate (a silence effect on a long cooldown). Basically, hit IF if you are in trouble.
You'll also have a few abilities that don't have rune or RP costs. These include your summonable ghoul, Horn of Winter (which should be up all the time and gives "free" RP), Empower Rune Weapon (resets all your runes, best for emergencies where you really need a certain ability RIGHT NOW), and Blood Tap (allows you to turn a B rune into a Death Rune temporarily; a more advanced move to master).
And that brings us to Death Runes. Death Rune can stand in for any of the other rune types. So if you have 2 Ds, you can, for instance, BS x 2, or use an FU strike, or IT+PS. They give you more flexibility and options, but they also define your rotation based on what spec you are in by allowing you to use your best move a bit more often than you can at base. The Death Rune talent in each tree, along with the new RP moves in the tree, give each talent tree it's flavor and differentiate their play styles. Here is a quick rundown of how each spec plays when leveling:
The Blood tree overall focuses on self-healing. It uses most of its RP on Death Coils, though it can summon a floating blade that copies the DK's attacks occassionally. This tree buffs Death Strike as it's preferred FU move, and replaces Blood Strike with the more powerful version, Heart Strike. HS does more damage and has a built-in aoe "cleave" effect. Many of the talents in the tree buff HS.
Blood differs from the other trees in that it generates Death Runes from it's FU attack. This makes sense when you consider that per rune, Heart Strike is buffed into being the tree's strongest attack. You want to get more B Runes in exchange for fewer F and U runes. When you use Death Strike, those 2 runes will regen as Death Runes, which can then be used for 2 Heart Strikes instead of another Death Strike.
So, after opening a fight with IT and PS, use Death Strike and 2x Heart Strike. Once those runes cooldown, you'll be able to throw out 4x Heart Strike and a Death Strike. After that, just use IT + PS whenever your diseases fall out, spend B and D runes on HS, and spend spare FUs on DS.
Here is a sample leveling Blood spec. Season to taste.
The best way I can describe the Frost tree for DPS is that it's focused on burst damage. It gives you a number of direct damage abilities, and buffs the hell out of their crit chances and damage. It will use its RP on Frost Strike, which is a supercharged replacement for Death Coil that can only be used from melee range. It also provides an alternative FU move, Howling Blast, for AoE situations, and is the only tree with an AoE CC ability (Hungering Cold, 40 RP).
For Frost, Blood Strikes are weak and really only used to generate Death Runes for your big FU strike, Obliterate. Normally, Oblit removes diseases on the target, but one of the Frost talents prevents this, making the move spammable.
So a Frost rotation would start with PS+IT, Blood Strike x2, Oblit, Frost Strike. Then you'd just spam Oblit x3 once all those rune regenerate (2 normal FUs plus 2 Ds available), FS. Rinse, repeat. No frills, just straight-up big-crit pwnage.
Here is a sample Frost levelling spec. Best served...COLD!!!
You want frills? Oh, Unholy's got frills, buddy. It's got a perma-Ghoul (your ghoul becomes a controllable pet rather than the passive companion it is for the other specs), a run speed increase, and a mounted speed increase. It can blow corpses up (thought I recommend you not spec into Corpse Explosion). It can summon a Gargoyle for big burst damage. A lot of your dps (20% ish) comes from your ghoul. You'll use your RP to put up Unholy Blight, an aoe damage aura (which is also better damage on single targets than DC), and then spend extra RP on Death Coils in between. This tree provides some protection in the form of Bone Shield and includes a replacement FU move, called Scourge Strike.
Don't let the low numbers on the SS tooltip fool you: it cuts through armor. SS is your heavy hitter, and you want to spam that bitch! To that end, like Frost, Unholy gives weak-ass Blood Strikes that serve only to generate Death Runes to fuel your SS-happy war machine.
The rotation is exactly like the Frost rotation: IT+PS, 2x Blood Strike, Scourge Strike, RP dump, followed by a hefty 3x Scourge Strike.
Here's a sample Unholy leveling build.
As you can see, the DK resource system necessitates dps "rotations" where you apply diseases, then generate Death Runes to set up your spec's heavy hitter, whether that be Heart Strike, Obliterate, or Scourge Strike. Unload DCs or FSs between rune cooldowns, and you'll mow down anything in your path.
You can also augment your performance through gear and glyphs. You can see recommended major glyphs for each talent tree by clicking the provided link to the talents. I recommend all levelling specs use Glyphs of Pestilence, Raise Dead, and Horn of Winter as their minor glyphs. As for gear, focus on Strength first, with Attack Power a close second. Hit and Expertise Ratings are excellent until you get around 8% hit or 6% expertise. Haste and Crit ratings are also strong. Armor Pen is OK, but not recommended for Unholy, and Agility takes up the rear, but isn't useless. Though many of your abilities appear to be spells, please note that Spell Power, Intellect, Spirit, and spell-specific buffs have NO effect on your attacks!
Those are the basics. There is a LOT more to playing a DK well, but this overview should get you off the ground (and out of the Thrallmar inn) and making headway through Outlands.
Enjoy, and Suffer Well, brothers and sisters of the Ebon Blade!