I use " marks because many talents serve multiple roles or may not appear as tanking benefits. Death Knights get the unique honor of being the only class with 3 different tanking trees. Because of that there will surely be a good variety of tanking specs used, and even where one may become the more universal preference, there certainly are no wrong choices yet. I won't belabor the dps talents that you will inevitably take to get all the talents you want (since DK's don't have high threat moves, you simply dps).
The Big Three:
These talents appear on the first tier of all three trees. In my humble opinion, you'd be a fool to not take any one of them.
> Toughness (Frost) = 3/6/9/12/15% increase armor from items, and 6/12/18/24/30% reduction to duration of movement slowing effects.
> Anticipation (Unholy) = 1/2/3/4/5% additional dodge chance.
> Blade Barrier (Blood) = 2/4/6/8/10% bonus parry chance, when you have no blood runes active, for 10 seconds.
Standard buffs, but they're in tier 1 and so simple and direct to mitigation and avoidance, they should be a mainstay regardless of your tree of choice for tanking.
Additional talents, organize by tree:
> Butchery (t1) = Whenever you kill a target you get 10/20 RP, and you generate 2 RP every 5 seconds. (I'll discuss RP generation value more below.)
> Bladed Armor (t2) = 1/2/3/4/5 AP per 180 armor value. (It was much more valuable as Strength, but it was deemed "too good.")
> Scent of Blood (t2) = 15% chance when you get hit with a physical attack, that your next 1/2/3 melee swings generate an additional 5 runic power. Cannot occur more than once every 20 seconds. (If you're tanking, this should proc pretty nearly every 20 seconds. If it did it would be 15 RP/20 sec or 3 RP every 4 sec. If we give a conservative 5 second window for not proccing onto the CD, it still amounts to 3 RP every 5 seconds which is 50% more than the passive granted by Butchery, and stacks, of course.)
> Rune Tap (t3-4) = 1 pt talent, for the cost of one Blood rune you restore 10% of total health, with a 1 min CD. With improved talenting it restores 13/17/20% of total health with a 50/40/30 second CD. (20% of your health every 30 seconds seems a little too good not to have at your disposal. Combined with Vampiric Blood that's 50% of your total health every minute!!!!)
> Spell Deflection (t4) = you have a chance equal to your parry chance to take 10/20/30% less damage from a direct damage spell.
> Veteran of the Third War (t5) = you gain an additional 2/4/6% Strength and Stamina, and an additional 2/4/6 Expertise. (All major buffing to tanks, and they increase the value of stacking the raw stats that you'll get by the truckload on tanking pieces.)
> Mark of Blood (t5) = places a Mark on the target that lasts 20 seconds. When the marked enemy hits their target they also heal that target for 4% of their total health. Lasts 20 seconds, has a 3 min CD. (You can make a case for the other direct healing abilities, but this is a major quick set buff that can amount to some serious numbers, especially combined with Vampiric Blood.)
> Abominations Might (t6) = your Blood/Heart Strikes have a 50% chance, and your Oblits have a 100% chance to grant all party/raid members within 45 yards +10% AP. Also gives you a constant 2% bonus to your Strength. (The passive is the major tanking buff, though the raid buffing is a big value to everyone and doesn't hurt your threat at all.)
> Blood Worms (t7) = your melee attacks have a 3/6/9% chance to spawn 2-4 worms that heal you for (double) the amount of damage they cause. (Like the other healing abilities, you can debate the value, but the fact that it is passive makes it all value since you don't have to work it into rotation.)
> Vampiric Blood (t8) = Increases the effect of all healing done to you (by your abilities, potions, and friendly healers) by 50% for 20 seconds, with a 1 min CD. (This is a little too huge for a tank, it's a roundabout way to make you a stronger tank, but it makes you very easy to keep alive for its duration. The drawback is, of course, that you are relying on healers, not your own innate survivability.)
> Will of the Necropolis (t9) = Reduces the CD of your Anti-Magic Shell (from 60 sec) by 4/8/12 seconds. When your health drops below 35% your armor increases by 10/20/30%. (Similar in effect to Protadins' passive survival talent, though through armor instead of static damage reduction.)
> Blood-gorged (t10) = When your health is above 75% all of your damage is increased by 2/4/6/8/10%. Grants 1/2/3/4/5 additional expertise. (I'm not a huge fan of the primary effect as a tank, since you can't rely on your health being that high, but the expertise and the occasional damage buff is almost certainly better taken than not, though you may not in favor of dipping deeper into a secondary tree.)
The tanking ability of Blood is best characterized by the healing buffs that ease the healing duty and the most basic of stat buffs that make your gear-granted mitigation/avoidance more effective. It actually plays better than it sounds.
> Unholy Command (t2) = reduces the CD on Death Grip to 30/25 seconds (from 35). (While this has obvious value for keeping strays and runners in order, I don't often spend the points here myself. That being said this is not a talent for DPS builds.)
> Ravenous Dead (t2) = increases your total Strength by 1/2/3%, and increases your contribution to your Ghoul's Strength and Stamina by 20/40/60%. (Any measure of % increase to Strength will be a good threat and avoidance buff, plain and simple.)
> Shadow of Death (t4) = Increases your Strength and Stamina by 2%, and when you die you are immediately rezzed as a ghoul to keep fighting for another 45 sec. (Again, Strength and Stam increases are valuable to base survival.)
> Magic Surpression (t6) = You take 1/2/3/4/5% less damage from magical sources, and your Anti-Magic Shell reduces the spell damage taken by 80/85/90/95/100% (75% without talents). (This in and of itself is a huge buff to tanking anything with spell damage, but it is also a precursor to Anti-Magic Zone, which is a major step up from there.)
> Anti-Magic Zone (t7) = Creates a static dome region in which all friendly targets take 75% less magic damage while inside. This lasts for 30 seconds or until the shield absorbs [10,000+2xAP] damage. Has a 2 min CD. (This is a terrific boon to a raid, whether or not you are actually tanking. It is a safe zone that can fundamentally alter raid encounter strats for your team.)
> Desecration (t7) = Plague Strikes have a 20/40/60/80/100% chance to create a zone centered around the DK in which enemies move 50% slower, and you get a 5% damage buff, which lasts for 12 seconds. (The slowing region is handy, since Unholy DK's have powerful AOE ability, and who doesn't like making it nearly impossible for runners to get away from you?)
> Bone Armor (t8) = Self-buffing shield with 4 bones. So long as at least one bone remains you get 40% damage reduction and a 2% damage buff. Bones are consumed by being hit like a Shaman's shield, and like a Shaman's shield there is a hidden CD on charge consumption (roughly 2-3 seconds). The shield will last 5 min if the charges are not consumed and has a 1 min CD. (This is Unholy's bread and butter for tanking survival. In most real tanking situations it can buy you 12-15 seconds of 40% damage reduction out of every minute. You'd be smart to keep on it though and only save it for emergencies when you have a fight you know you'll need extra protection.)
> Rage of Rivendare (t10) = Increases all damage to targets afflicted with Blood Plague by 2/4/6/8/10%, and grants the DK a passive 1/2/3/4/5 Expertise bonus. (Just like Blood-Gorged for Blood and Tundra Stalker for Frost, the threat bonus is foolish to not have, but the Expertise is just quality survival.)
You will notice that Unholy has very little offerings to survival in and of itself. Instead what the tree has to offer is fantastic AOE threat, maybe the closest rival to a Prot Paladin. Bone Armor can go a long way to improve your Survival abilities, but you have to be smart with you supporting tree(s). Magic Suppression and Anti-Magic Zone make Unholy Knights strong spell damage tanks, and great potential off-tanks for any raid.
> Improved Icy Touch (t1) = Icy Touch does an additional 10/20/30% more damage, and your Frost Fever reduces the targets melee attack speed by 16/18/20% (14% untalented). (I wouldn't assume every DK tank should take this, there are other ways to ge the same de-hasting, Tclap from warrior among others, though Frost Fever is very easy to keep up. This will amount to good mitigation, and is a strong support to any tanking build.)
> Lichborne (t3) = You become undead for 15 sec, while undead you are immune to Charm, Sleep, and Fear, and all targets have an additional 25% to miss you. 3 minute CD. (It has a lower potential for uptime, 15 sec out of 3 min, but as an emergency button, 25% avoidance is huge for one talent point. It is low enough in the tree that most tanking builds can take it.)
> Runic Power Mastery (t4) = Increases your maximum Runic Power by 10/20/30. (While not directly buffing any survival, the increased RP reservoir allows for a bit more leeway in your rotations and a better chance of having the RP for Icebound Fortitude when you really need it.)
> Frigid Dreadplate (t5) = Anyone who hits you gets a debuff reducing their chance to hit you by 2/4/6%. (Point for point this is the single best avoidance talent available to DK's. However there is chatter from Blues that it will be brought down to 1/2/3% in a future build. Still it is pure avoidance, every bit as valuable as Anticipation.)
> Endless Winter (t6) = Your Chains of Ice have a 50/100% chance to apply Frost Fever to the target, and your Mind Freeze costs 10/0 RP (untalented it is 20). (This is one I admittedly leave off, but the value of a free interrupt is pretty tempting. It is also the pre-requisite for Chillblains which some tanks may want for obvious reasons.)
> Chillblains (t7) = Targets afflicted with Frost Fever are slowed by 30% for 10 seconds. (The potential for keeping pulls under control is nice, and it's always pleasant to be able to slow targets as need, but I've always seen it as investing in things going badly. It has value to be sure, but I expect this will be a more selective choice.)
> Frost Aura (t7) = All party/raid members within 45 yards of the DK receive 0.5/1 spell resist per level of the DK (40/80 at level 80). (Like Anti-Magic Zone, this is a valuable buff for the shear survival of a raid. 80 resist won't negate anything, but it will certainly take the edge off.)
> Unbreakable Armor (t8) = When activated this self-buff increases your armor by 25%, your Strength by 10%, and your Parry chance by 5%. Lasts 20 seconds and has a 1 min CD. (This is close in comparison to Vampiric Blood and Bone Armor, by the balance of it, but I'm rather fond of this one in particular. Why? It has a good measure of uptime, 20 seconds out of the minute, and it is a solid fundamental increase to your standard survival tools and a nice threat buff to boot. This is one you use on CD unless you're trying to time a particularly rough patch in a fight.)
> Guile of Gorefiend (t9) = Increases crit damage bonus of some of your primary threat moves by 15/30/45%, and increases the duration of your Icebound to 14/16/18 sec (12 untalented). (This increases your uptime potential for Icebound Fort to 18 seconds out of the minute which is a big enough deal. That goes from 20% to 32% uptime at 50% damage reduction.)
> Acclimation (t9) = When you are hit by magic damage you have a 10/20/30% chance to get an 18 sec boost to your resistance of that magic type. Stacks up to 3 times. (This is a very useful buff if the tanking environment will involve some consistant sort of spell damage. It may be a bit more situational though, so it may not be a full-time staple talent.)
> Tundra Stalker (t10) = Increases all damage you do to targets afflicted with Frost Fever by 2/4/6/8/10%, and increases your Expertise by 2/4/6/8/10. (Like its equivalents, the threat buff is silly not to take, but the Expertise, which is double the equivalent talents in the other two trees, is a great survival buff.)
> Hungering Cold (t11) = Freezes all enemies in 10 yards in ice for 10 seconds. Any damage other than diseases will break the target free, otherwise they are completely incapacitated for the duration. Costs 60 Runic Power and has a 1 min CD. (This is the one 51-point talent that I actually feel is worth mentioning for a tank. That being said it is essentially just a handy tool for a larger pull going wrong, or an emergency where you want to shut things down for a moment.)
Frost is best characterized by its middle of the road base tankability. It has some abilities for spell tanking, some for physical tanking, and has the potential for the best control tanking (with slows and HC). Another angle that I favor Frost for is that your main threat spam are: Icy Touch, Howling Blast, Frost Strike, and the odd Obliterate. The first 3 which will be the most common used are all spell damage, ignore armor completely, and cannot be dodged/parried/blocked, meaning your only parry-haste liability is your auto-attacks (which, unique to DK's, can be with a slow 2-hander, meaning far fewer swings to worry about).