I mentioned in my last post that there are tanking options in every talent tree, but that you wouldn't want to use exactly the same spec for tanking that you do for DPS. So what sets a tanking spec apart from a DPS spec when they're both in the same tree?
The Big Three
Each tree has a first-tier talent that's a must-have for tanking: Blade Barrier in Blood, Toughness in Frost, and Anticipation in Unholy. Once you've filled out your chosen tree to the 51-point talent, these should be your next priority if you plan to tank while leveling. No level 80 tanking spec should ever be lacking these talents! You can use these as a cheat sheet in PuGs to see if your DK tank is actually a tank and not just a DPSer who wanted a faster queue. These talents, in addition to a set of proper tanking armor and a tanking-appropriate weapon rune, are part of what keeps you from going squish like a clothie.
The A La Carte Talents
Each tree also has at least a couple damage-mitigating talents far enough into the tree to make them prohibitive to pick up unless you're committing to that tree. Frost has several: Frigid Dreadplate, Unbreakable Armor, Improved Frost Presence, and Acclimation. Lichborne used to have a damage-mitigating component, but it's since been nerfed into something more suitable for PvP than tanking.
Blood's tanking talents are less directly mitigation-based. Scent of Blood doesn't mitigate on its own but procs when you mitigate. Veteran of the Third War gives a Stamina boost. Rune Tap, Improved Rune Tap, Vendetta, Mark of Blood, Bloodworms, Improved Death Strike, and Vampiric Blood are all self-healing talents. Spell Deflection and Will of the Necropolis are the only real mitigation talents in the tree. Improved Blood Presence should be skipped in a Blood tanking spec as you will still tank in Frost Presence (which I'll be writing about later.)
Unholy has the fewest talents with tanking utility, but Magic Suppression and Bone Shield do provide some mitigation. Again, Improved Unholy Presence should be skipped in a tanking spec because you won't be tanking in Unholy Presence.
Keep in mind that you may not need to use every tanking talent in your chosen tree, as depending where they're at in the buff/nerf cycle some may not be worth taking at the moment. And Death Knights can still do acceptable DPS out of Frost Presence with a tanking spec - not competitive with a proper DPS spec in raids, but good enough for soloing. When leveling or even in PvP you might want to put some points into these tanking talents for survivability.
Next I'll be explaining the Presences and why the best Presence may not match the tree you're specced into!