Ok, so after tanking for the last 2 weeks in heroics, 10 man Naxx, and Sartherion (starting 25 man Naxx tonight) I think I have worked out the spec I am going to use as my main tanking spec. I haven't currently tried it, but in theory it should work just fine.
Some notes: I am an OT. the MT is a warrior. I find DKs to be amazing OTs since our AoE threat capabilties are awesome, our gimicky magic mitigation is also useful in certain situations, and the way our damage reduction works is great for short bouts of tanking (where you taunt off the MT for a while) even moreso than for full-on tanking.
Anyway, I tried Frost a while back, didn't like it much. UA is just not enough damage reduction at basically 10% parry and armor for 20 seconds. It doesn't work against magic, which is what a lot of the Frost tree is about, and must be cast in combat (good bye Frost rune).
So I have been using Unholy, since it's AoE threat generation is far superior and is great for 5 man heroics, and Bone Armor is the win.
Here's the spec I'm going to use :
Unholy Build
And here's the breakdown in my reasoning:
Unholy:
First tier:
Obviously take everything. Dodge, Morbidity, and Vicious Strikes all good.
Second tier:
Epidemic: I have run with this and without... it really depends on preference. Running without isn't bad, but requires re-applying diseases every rotation near the beginning. I don't have it currently, but think I will spec back into it so I have more options of BB/DnD early in a rotation. However, these points could arguably be moved.
Ravenous Dead: 3% extra strength is nice, coupled with Shadow of Death you are running 5% extra Strength. I have somewhere around 900 strength base in my tanking gear, which means about 27 extra str from 3%, which is nice imo for 3 points. But again, these could possibly be moved into Virulence. Still debating on this one.
Virulence and Unholy Command I didn't take - Virulence as stated can be argued for, and UC I just don't find to ever be needed.
Third tier:
Necrosis: As noted before, can be great threat even on RS procs.
Corpse Explosion: Didn't take because I never find a good use for it (I hate any abilities that take a single F or U rune that aren't IT or PS).
Outbreak: Don't find to be a good investment of talents for abilities you use max once every 10 seconds. Could be argued for though.
Fourth tier:
Shadow of Death: the only thing I took here for the 2% stam.
BCB: As noted before can be parried, which is not worth the extra threat imo.
On a Pale Horse: How I love this talent for flying around in Northrend, but unfortunately doesn't help tanking. Reduction on stun and fear is negligible imo.
Fifth tier:
Gargoyle: skipped of course.
Impurity: Great for threat.
Dirge: I do love me some RP, but this could arguably be moved if preferred. I just didn't see anything else to be better than as filler. If anyone is so inclined, the points could be moved into Frost as lead to Lichborne, or into Two-handed Weapon spec for threat. I like them here though.
Sixth tier:
Master of Ghouls: I don't bother with this anymore. Ghoul pet w/o Avoidance dies way too fast, and the stun is minimally useful. As a tank, the ghoul is just more a liability than anything.
Magic Suppression: Here's the debatable part. I made room for the extra 6 points to get this and AMZ. I agree that the talents are very gimicky and limited in usefulness - however, I have tanked all in Naxx 10 man except the final 2 bosses, and I must say that having Magic reduction talents is a must have. Sure, it's not useful EVERY single time, but at least half the fights in there I am spamming my magic shield on CD. Since I am not frost and do not have Acclimation or Frost Aura, I would say that making these 6 pts available as Unholy, if you plan to do real tanking, is not a bad investment. Nice thing about this talent is at least it has the 5% reduction on magic damage all the time, which I assume stacks with Bone Armor's 40%.
Reaping: Mandatory. Death runes are great, and if you keep your Death runes, *they don't count as Blood Runes* so that Blade Barrier stays proc'd more often.
Seventh tier:
Desecration: I was tanking with this, since 5% damage seems like a good amount of threat. However, I find the slow to be more of a liability - there are many times as an OT where you want to move things fast (the fire adds on Sartherion, the blobs on Globbulus, etc) and the slow just hampers your ability to do this. Also, the animation is annoying, and tends to impair visibility of other ground effects that bosses may be casting, and causes extra lag. So in the end, I feel this is not a good or necessary tanking talent, and 5 points can be much better spent elsewhere.
AMZ: This I think, yes is gimicky, but can come in very useful, for fights like the 4H. Using it on Thane's Meteor would be nice, and on bosses like Gorthic it can be used as more mitigation against his magic spam. Overall, I think the 6 pts to get these are fully justifiable, since I have encountered a ton of magic damage inside of Naxx and many heroics. I also don't feel I need these 6 pts for anything else.
Unholy Aura: debateable, but I like to take it, since mobility is always a good thing. However it could be skipped.
Eighth tier up:
I think these are all self explanatory. Night of the Dead and Wandering Plague are useless as a tank. Everything else is golden.
As to my sub: Blade Barrier and Toughness of course. And I always like to take Bladed Armor as Unholy because so much of our threat is AP based, and from this talent I gain about 700 AP alone. If you skimp on this talent at all, you better be getting some damned good threat elsewhere.
And unlike most people, I do not waste the points to get to Lichborne. Wasting 2-5 talent points to get a 3 min minimally useful CD is not my idea of a good spec. Between Bone Armor and IBF, I have all the mitigation I really need, and 1 extra long CD will not save my ass. As for the fear immunity, a 3 min fear immune will not help you. If fear or w/e is an issue, then it will be happening more than every 3 mins, in which case On a Pale Horse will be more beneficial.
You could make arguments to get Lichborne as Unholy, but I feel that it is a waste for Unholy and is better for Blood or Frost specs, since they do not have quite the demand on talent points. I honestly feel having the "gimicky" magic reduction from MS and AMZ are way better than a 3 min CD 25% miss buff. But again, it's a lot about preference and what you are tanking.