What to pick

[Nerdanel]

I've recently started a dk as well to have a bit of a fallback if I get bored of mage dps. Only thing atm I can't seem to figure out is wheter it's possible to tank in blood spec. It's great for leveling and seems to be able to give you enough surviveablity when it comes to taking on multiple mobs but can't seem to find a straight answer wheter or not it's a usable tanking build.

[Daffyd]

All three DK trees have viable tanking builds but they look nothing like the DPS builds.  I believe the general consensus of opinion is that Blood is the way to go for raid tanking but I still have half a level to go with mine before I get to test it out.

[Dar]

All three trees are indeed able to tank, however unholy is a bit left behind.
Blood and Frost are both very usefull in raids. I just threw away my dps spec and now have a blood spec for HP and survivability on hard hitting bosses (more health from the tree and the 1handed weapon + Vampiric Blood) and being able to hit just a tad harder when you are not tanking thanks to Hysteria.
I still have, and use, my dual-wielding frost tree, it gives less health (2 1handers gives less stamina and no slots) but also more avoidance. Most importantly they give alot of AoE threath, something blood is very poor on. The use of a Killing Machine proc + a Howling Blast gives you a 8k crit on all mobs around you.

To summarise:
Blood: HP, Self Healing, great single target threath, being able to enrage yourself or a teammate
Frost: Dualwielding awesomeness, fasthitting (more Runestrikes), Enormous AoE threath, Avoidance (more Runestrikes)

If you are seeking good builds, these are my builds and they seem to work pretty damn well (with the frost one i tanked ICC upto and including Festergut)
Blood
Frost

[Nerdanel]

Once I hit 80 this will mean some serious respeccing. Have to admit the frost version looks nice

[Phred]

Completly offtopic but still related ... the questionary subject made me think Warrior instantly