I'm now going to review the specs I've used since patch 3.1 hit and talk about the way I use them.
This spec double dips Frost for Frigid Dreadplate while taking many of the survivability and self-heal Blood talents. I think it is the strongest possible DK survivability spec. It has fantastic cooldown uptime, great mitigation and outputs in 25 man raids about 1/4 to 1/3 as much healing as a full-on healing specced raid-buffed healer. I use this spec when I'm tanking a boss that might kill me and aoe threat is not an issue.
Rotation is diseases + Heart Strike, Death Strike, Rune Strike. Excess rp is dumped into Death Coil self-heals (using Lichborne) or Death Coil attacks.
Mark of Blood (20 secs every 3 mins)
Icebound Fortitude (12 secs every minute)
Anti-magic shell (5 secs every 45 secs)
Vampiric Blood (30 secs every 2 mins)
Rune Tap (instant every 30 secs)
Lichborne (15 secs every 3 mins)
Trinket: Defender's Code (20 secs every 2 mins)
I aim to keep most of these on cooldown while tanking in this spec. Vampiric Blood is the most powerful cooldown so it's the one to plan the others around if you know the boss damage will spike or the one to use first otherwise.
Threat tanking build (Unholy)
This is significantly lighter on survivability but gets a number of area effect threat abilities as well as a well-developed Scourge Strike. Scourge Strike and Death and Decay are especially important in a high threat build because they have an extra threat multiplier in addition to the multiplier gained from Frost Presence. (Source: Tankspot).
Rotation for AoE threat is:
Death and Decay, diseases, Pestilence, Unholy Blight (as soon as runic power permits), Blood Tap, Blood Boil. Then Scourge strike, Rune Strike + Bloodboil the skull, adding Corpse Explosion and Unholy Blight when available.
Rotation for single target threat is:
Death and Decay, diseases, then mash the Scourge Strike, Blood Strike, Death Coil keys with Rune Strike macroed.
Note that there are no pet talents in this build. Pets do not give you threat, they have their own separately tracked level of threat.
Single target rotation: diseases > scourge strike, blood strike, death coil
AOE rotation: death and decay, diseases > pestilence, and as many unholy blights, corpse explosions and blood boils as you can squeeze off.
Keep Empower Rune Weapon, Gargoyle, Blood Tap and Ghoul Frenzy on cooldown.
Pretty straightforward nuke build, the pets are important and pet uptime is a big factor. Even without especially good dps gear I was around 6th-7th in our 25 man raids with this spec.
My main interest is playing dual tank spec and I constantly spec switch. My Blood tanking spec is much better on dangerous pulls or if we are doing a fast paced raid and the healers are struggling to keep up with us. The threat tank spec is essential for collecting adds on fights like Sarth with drakes. Always judge from pull to pull which spec works best. It's great fun to drop in and out of the two different specs to handle different challenges and I'm having a blast with dual tanking specs.