Now that we have a general idea of how to prepare to tank and what you do once you're in a group, let's look at some more specific tanking builds. You may notice that most of my builds try to grab Virulence on the second tier of Unholy. This is because, especially if AE tanking, you'll be casting a lot of spell-like effects. I find having that extra +hit is imminently helpful in making sure I do my damage and grab my threat. Likewise, Blade Barrier finds its way into all my tanking builds. 10% parry on a first tier talent definitely isn't too shabby, but you have to spend the blood runes to activate it, whether that be a couple Blood Strikes on a single target, or a Pestilence/Blood Boil combo for an AE attack.
Tanking Talents: Frost
Frost ends up being a tree with high mitigation and lots of very nice defensive tools. Acclimation and Frost Aura will provide excellent anti-magic support. Lichborne and Unbreakable Armor give you some extra buttons to push to weather the real storms, while Guile of Gorefiend also provides a nice buff to the baseline Icebound Fortitude for even more protection. Howling Blast and Hungering Cold should work their way into your damage rotation, bringing the freezing power that should keep the mobs glued to you.
Here's a level 80 build for Frost, while this 70 build should get you through a few Nexus or Utgarde Keep runs if you so desire.
Tanking Talents: Unholy
Unholy's advantages come in the form of Bone Shield and Anti-Magic Shell, as well as a whole plethora of extra AE skills such as Wandering Plague, Desecration, and Unholy Blight that should allow you to keep aggro in AE situations almost without trying. If you don't mind essentially wasting an extra 5 points in Frost, Lichborne is also a good compliment to an Unholy tank build to fill in the gaps when Bone Shield is down.
A level 80 build might look something like this, while a level 70 build could work out like this.
Tanking Talents: Blood
Blood actually doesn't have quite as many unique bells and whistles for tanking as the other two trees, it seems, or rather, the form they take is a bit more passive. Spell Deflection, Will of the Necropolis, and Blade Barrier both provide nice, easy to trigger random bonuses that should go off often enough that you won't feel like winning a battle is luck based.
As far as triggered things go, Rune Tap and Mark of Blood will probably start feeling a bit low powered on boss battles, but Vampiric Blood should still hold up. On aggro holding, though, you're essentially on your own with the baseline talents, although they should be enough in theory. Grabbing Morbidity from the Unholy Tree should definitely help there.
Here's a Blood build at level 80 that grabs Lichborne from the Frost tree for some extra protection, and here's a slimmed down level 70 version for attacking Utgarde.
Go forth and Conquer
As always, my Death Knight advice comes with a disclaimer: We're still working on it. We need years to catch up to other classes in terms of knowing what we can do and what talents work best for us. It may be that once we've had some time in the upper raid content, we'll understand better what talents really do work for raiding, or what type of weapon is best for raid tanking, for small group tanking, and so on and so forth. Until then, welcome to the frontier, pioneers. We're charged with making sure the great tank shortage of Burning Crusade is not repeated, and so far, it looks like we'll be able to pitch in and do our part.