I've only tanked Heroics, did my first one within an hour of dinging 80. I simply cannot see why Frost is better than Unholy for tanking.
As of dinging 80 I had 40% avoidance between parry, dodge, and miss. Add in Blade Barrier and that's up to 50%~. Pretty good for Bone shield.
What it comes down to really, is Bone Shield vs. UA. Everything else is negligible or easily gotten by any spec (i.e. Toughness).
UA does nothing for spells, and I don't think the math works out to 40% mitigation for the duration.
Bone Shield has 3 second internal cooldown. With the glyph (you *are* glyphing it, right) it lasts 15 seconds minimum. It really ends up lasting longer than that depending on avoidance. Also, you can pre-cast it at the beginning of the fight and chain 2 at the start, something you can't do with UA.
So with Frost, you have 38 seconds of guarenteed mitigation (how much from UA I'm not positive on). With Unholy, you have 27 seconds of guarenteed mitigation, minimum 40% reduction, with the possibility of up to 100% uptime.
Keep in mind that magic damage is a large part of many bosses. For example, the first Heroic I tanked was UK, and all 3 bosses have magic damage, and we would not have been able to down the final boss without AMS and AMZ (his shout is shadow damage lawls).
As unholy you also provide good buffs (UH aura and Ebon) that cannot be applied by any other class, as opposed to the lolwindfury Frost buff. 6 points to have a shaman buff is a big waste imo.
The AoE threat is better as Unholy, the magic mitigation destroys Frost, and with bone shield you have equal to or better than mitigation than Frost.
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