Trm, you meant Will of the Necropolis, I guess? Might of Mograine is a strong talent aggro-wise only.
Frost has a bit more mitigation from Frigid Dreadplate and Improved Frost Presence. Acclimation is reliable on fights with continuous spell damage so the talent can stack up and remain stacked. The problem is that there aren't that many fights, most notably LK's transition, Kele tanking and Sindragosa. Festergut's aura damage is already mitigated by spores, PP's damage is healable, Rotface never chooses to spit on the tank, BQL's air phase is easy enough with AMS, Lady's Frostbolts are too rare to stack up the talent.
But that's talking about ICC - there are plenty of bosses before that on which Acclimation is very useful - Sapphiron is a clear example on which you get a fully stacked Acclimation through the whole fight. Some VoA bosses are also a very good opportunity. And of course there are a lot of heroics where this talent plays a big role.
I am very willing to tank with Unholy once I get a 80 DK on Lordaeron. I've done it before and I have been very successful at it. Bone Shield is absolutely insane with sufficient avoidance - that's like a constant Icebound Fortitude on me as long as I can manage it correctly and have enough avoidance. And then we have the magic reduction from Magic Suppression and the amazing raid tool which is Anti-Magic Zone.
At the end of the day, once we reach ICC, Blood will be the best spec - there's no doubt about that, that's pretty clear from the way the spec works with the ICC buff and how much the other two specs suffer from the debuff. But before ICC I'd say we can play around with Frost and Unholy and be just as successful, if not even more successful than Blood.
P.S. - Regarding Frost weapons - Broken Promise is everything you could ever wish for at this content as a FDK tank. 2.50 speed, +64 stamina, defense, hit and expertise... I mean, seriously WOW. And it's not Unique. And it sheathes on the back. And it's priority for DKs over Paladins. Warriors may also want it, but it's still best for DKs. I remember when I got it on my retail DK and I was like "oooooohhh myyyyy god".
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Then also you have this beauty from UK Hc and for a start - this axe from this quest in Zul'Drak. These weapons, together with Broken Promise, are more than enough for the current content.
Apart from that, the only reason you would want to DW fast tank weapons is if you're below the def cap. Some people also advocate for a DPS MH and a tank OH so you can retain the high threat while also having some defensive stats. At the end of the day two DPS weapons will yield the highest threat.

