This is my first Mechanics post. These posts will explain how to tank as a DK for beginners, because I'm tired of people assuming I'm a bad tank because of the rest of you.
Threat, for those who don't know what it is, (and trust me, I've seen plenty of tanks that don't know what it is) is a hidden number that each mob you face keeps track of for all players fighting it. Whenever anyone attacks the mob, or heals one of its enemies, this number goes up. Spells like a rogue's feint, pets' cower, or a priest's fade reduce this number. When a player dies or uses a spell like vanish, their number on the list is removed, and will restart at zero if they reenter combat. Whichever player that mob has the highest threat number for is the player they will attack.
Frost Presence increases all threat you generate by 45%, always tank in frost presence.
As a beginning death knight, you cannot tank successfully until you reach level 60, at which you learn your most important threat spell ever: Death and Decay. The short story is, cast Death and Decay early and often, and preferrably under where the enemies are going to be standing. Mastering D&D alone will catapult your threat gain to newfound levels of competence. It has a very sizeable threat bonus attached to it, covers a decent area, and can be targetted at a range. Furthermore, it can be improved with the tier 2 Unholy talent, Morbidity, which can increase its uptime to 66%, up from 30% for a mere 3 points (assuming that all sane DK tanks already have 5 points in Anticipation.) I can strongly recommend Morbidity to all DK tanks. In addition, the glyph of Death and Decay increases its damage (and thus, its threat) by another 20%. Also, D&D (like most DK spells) scales its damage to your attack power, so all that Strength on your gear is put to even more use.
D&D does have some problems, which you must compensate for with your other abilities. One of the most obvious problems is that, even talented, once cast, you cannot use it on something elsewhere for the next 15 seconds. Should a single mob break free of your Death and Decay, Death Grip is the perfect answer, not only gaining maximum threat on the mob, but also relocating it into the D&D cloud, ensuring that you will remain on top of its threat. In the event of a small group of adds, pestilence off your main target, or Howling Blast for frost tanks can serve as a great way to fill the space of time when you can't get D&D to cast.
The other obvious problem with D&D is that it requires 1 Unholy, 1 Frost and 1 Blood rune. In onrder to cast it, you must make sure not to ever use two of the same rune in the final 8 seconds before the spell refreshes. During long boss fights, you will have enough of a threat lead that missing a D&D (or indeed, even not bothing with it at all) is not a problem, but when tanking trash with the possibility of adds, you must carefully ration your runes to make sure your D&D will be on the ground as frequently as possible, because if you do get adds, you can only really be sure your threat is secure when they've stood in your Death and Decay.
If D&D is the meat and potatoes of your threat arsenal, then diseases are the fires that do the cooking; without them, including the meat in your meal will soon be a painful experience. D&D kicks your threat into high gear at the start of a fight, or when adds first aggro, but diseases keep you at the top of the Omen display so you can pay attention to incoming adds, burst damage and other hazards. Diseases currently do a very decent hunk of damage, and are getting another 15% buff in 3.2. When combined with the threat bonus from Frost Presence, they can free you to devote attention to other mobs for 10 global cooldowns at a time, giving you plenty of breathing room. To gain the maximum benefit from your diseases, be sure to cast Pesilence about as often as D&D, and be sure Blood Plague and Frost Fever are on your primary target before you do so. The glyph of pestilence increases the range of pestilence and only costs a minor glyph slot and a few gold. The glyph of disease is a bit more expensive, but refreshes your diseases on your primary target when you Pestilence, freeing up a frost and unholy rune, perfect for a Death Strike to help heal yourself with, or take the edge off burst. It also frees up two global cooldowns, which would otherwise be used to reapply your diseases.
So, if you are beginning down the road of DK tanking, here is some homework until next time. Try to master the use of D&D in securing threat on new mobs. Make sure all 10 seconds worth tick out on the mobs at the start of a pull, and keep it ready to control new adds.
In the next Mechanics post, I will go over the basics of defensive cooldowns.