I've actually run into very serious threat issues as my DPS players have been gearing up a little faster while my threat hasn't scaled as well. While my avoidance, mitigation, and health have increased throughout encounter teirs, my threat has not. Each set of nerfs has decreased it. ToC tanking drops are decidedly lacking in Hit rating and the upgrades that give significant Strength or EH increases cost yet more Hit and Expertise. I'm having to deal with boomkins that refuse to spec into their threat redux talent (and have no threat dump), super-spiky fireball mages, rogues that Tricks each other for the damage increase instead of tanks, and our paladins almost never use Hand of Salvation. My threat can float around 5.5k tps, but that just doesn't cut it up against the dps in guild. The gear options are problematic, as every time I'm close to one-handers at current content, someone shows up or whores them, preventing me from wanting to try DW with 170-ish dps weapons.

So I am noticeably concerned with threat. A tank that dies can have healing strats changed. A tank that threat-caps dps is another matter.

@Gelsk: If you can get an arena rating that high, I'd say go for it if you need the stamina.
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I've noticed in the OP that the build for the Unholy tank spec is a different one than the current live build. Would this cause Unholy to play worse off than Blood at the moment?

I thought some attention should have been brought to the error.

This - I've been doing this and even given the wasted stat of resilience it still generally ends up being a phenominal 2h tanking weapon for us.

In terms of just the sacrifice, I suppose it's a nice alternative if you really want to spec unholy (I personally find no use in having AMZ for current PvE content, but the magic debuff is handy), but if you're loving Blood's DR anyway, it seems kind of unnecessary.

Death Pact:
40% every 30 seconds, costs 40 RP

Improved Rune Tap:
20% every 20 seconds, costs 1 Blood Rune

Used with Vampiric Blood, gives 37%, allowing 15% over maximum if for some reason you're using it with more than 80% health.
Vampiric Blood is on a minute cooldown, so assuming you use them at the same time when both are up, IRT is an average 28.5%, which should be plenty while tanking... if your healers can't keep you up after you're popping 30% health back every 30s, something is wrong.
EDIT: I was mistaken about the RT glyph... it multiplies the 10%, so your previous 20% from IRT becomes 22%, with VB is averaged to 29.7%. Not much of a difference, but the 10% party heals may be useful.

This is just taking into account the health gain, not the ghoul's damage while it's up (minimal) or the rest of the unholy spec, or the fact that IRT is one extra talent point vs. NotD + Permaghoul

All that said, I'm a frost tank. My healers love the mitigation, my dps love icy talons, and everyone loves not having loose adds. Plus I can respec into HC/Chilblains when I need to slow/freeze. (FC + Anub ToGC).