How to Tank with a Death Knight

As a Death Knight tank, you're probably not going to get the respect that a tank of many other classes would recieve. However, you can easily make up for this by making a good reputation for yourself. There are many important things to consider when it comes to tanking.

Gear
When it comes to gear, defense rating should always come first. Make sure to use your Stoneskin Gargoyle rune on your weapon to increase your health and defense. If you're level 80 and want to tank heroics or raids, you should raise your defense to 540 through gear, as this is the cap and if you're not at it, bosses can crit and one shot you. After you are already geared for the defense cap, you're going to need to gear for both stam (for survivability and ease on the healer) and strength (to increase dps for threat, as well as parry for avoid). You can always tweak gems and enchants to maintain having the defense cap in the event of a gear replace lowering your defense.

Spec
Your choice of spec will vary, as any spec of a death knight can manage to tank quite well. However, there are talents in any tree that are definitely important. Blood Barrier in the blood tree, the armor bonus from the frost, and the anticipation from the unholy tree. After you've already picked up those three talents, I recommend getting ALL of the talents on the second line of unholy, perhaps skip the strength. These will help you keep your diseases and death and decay up more frequently.

Unholy
Unholy does very well with magic damage mitigation, through the use of several talents, including the almighty anti-magic zone that will protect everyone within the large bubble for 10 seconds. Unholy also has a speed increase while still in frost presence, which is of great use to a tank in many fights. Bone armor increases damage mitigation, and the AoE threat generation is quite effective.

Blood
Blood seems to be the best at survival, especially while tanking heroics where you only have one healer and the problems can start happening quite quickly. You can heal yourself in emergency situations, or pop abilities like vampiric blood and greatly increase the effect that the healer's heals have on you. Blood is decent threat generation, especially against 2-3 targets. You may have trouble with holding adds and what not with blood spec, but the survivability is nice.

Frost
In my opinion, it is easiest to tank with frost. You get "free" crits all the time, and if you use this crit on your howling blast, you can cause some amazing aoe threat. Through frost and rune strike, you can maintain threat upkeep better than any other spec. This is actually the only spec tree that is able to succesfully tank without ever dropping the Death and Decay threat AoE, unlike the other 2 tanking builds.