Woah, woah, woah! I came here to look for interesting replies and maybe help a noob DK player out and what do I see?
- You don't need glyph for taunt.
- Unholy is not designed to be a tanking spec.
Holy ****ing ****...
Taunt is a spell and as such it can miss. Missing your taunt means raid deaths and possibly wipe unless whoever aggroed has a threat wipe/stall (Ice Block, Fade, Feign Dead, Vanish, Soul Shatter) and is quick enough to use it. If you're playing a tank and you seriously think that glyph of taunt is a bad/useless glyph, please tell me your in-game characters so I can blacklist them for my guild's recruitment, thank you in advance.
Unholy is capable of tanking, just like Blood and Frost. In fact, it actually has the highest EHP of all 3 specs due to Bone Shield. The reason why Blood is better than Unholy, however, is that in ICC and RS you get -20% dodge, which hurts Bone Shield uptime greatly, and in ICC you get +30% max HP and healing, which helps Blood the most out of all DK specs.
"Wait. So Blood is only better than Unholy in ICC and RS?" - Yes. Outside ICC and RS Unholy and Frost are just as good, if not even better than Blood. No, that's not my opinion - that's a fact.
"So why do people only play Blood, then?" - because it's the easiest, it can solo stuff when overgeared due to self-heals, it has Hysteria for your donor rogue/hunter, and it is the end-game spec for ICC/RS.
"Just that?" - Yep. Blood has trash AoE aggro, the lowest avoidance and the lowest mitigation from the 3 specs, and its AP aura is provided by MM hunters (compulsory for every raid) and Enh Shamans (yeah, I know Enh Shamans are a myth). So there's no other reason you'd prefer Blood, really.

