I found this which may explain why during a couple of Heroic instances last night I was having so much trouble with damage mitigation. Poor Forian was almost pulling his hair out!
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The ever-helpful Ghostcrawler hit the forums late yesterday with a slew of changes to Death Knight tanking in the form of both bug fixes and buffs. The single biggest "buff" is actually a fairly significant bug fix; Death Knights had half the untalented, ungeared dodge of a Warrior or Paladin, and that was definitely never intended. Between that and a change to Blade Barrier (it's currently activated with all runes on cooldown; it's being changed to activate with only Blood runes on cooldown), Death Knights should see a significant improvement to their avoidance. Threat generation is also getting a nice boost, as Blizzard recognized that Death Knights suffered badly whenever key moves failed to land. Rune Strike is becoming a reactive ability like the Warrior's Revenge, and Frost Strike can no longer be dodged, blocked, or parried. Death and Decay has also been changed to be more competitive with Consecration and Thunder Clap, which is consistent with the overall trend toward AoE tanking effectiveness.
I've healed a number of Death Knight tanks in 5-mans now (you'll be hearing from my grumpy self about this soon) and recognized a few early versions of these issues, certainly in the form of Blade Barrier's often-spotty uptime. While I'm glad that DK's are getting more consistent threat generation, I have to admit that my real concern is the amount of burst they seem to take (something others have noticed as well), so I'm keeping an eye on the tweaks being made.
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This may mean that until the bug fix is applied I can't tank HC's.. half the dodge of an untalented naked warrior is a huge problem in an HC instance!
So guys, the good news from last night is that we were fighting a quite major disadvantage, undoubtedly with better gear this wouldn't be a problem but going in with such a handicap and borderline gear we did incredibly well. I must say finding this info has cheered me up by a huge amount, I was ready to hang up my sword this morning...