Looking at it right now for a Paladin vs a Blood DK, we seem to have:

Health: (1.144 vs 1.092 scaling, 0.052% difference for ~ 1.2k health (120 stamina) so equal at 2307 stam roughly then Paladin A)
Paladins: (base+x)*1.08*1.06*1.10
Death Knights: (base+x)*1.06*1.03*1.10

Paladin: Armour*1.10
Death Knight: (Armour - Shield)*1.10*1.6, so 1.6x(armour-shield) ~= shield

Static DR: (inc sanc on both)
Paladin: (0.94*0.97*0.97*0.97) = 0.858 multiplier on damage taken (think this is right)
Death Knight: 0.95*0.95*0.97 = 0.875 multiplier on damage taken

So Paladins take 98% of the damage DKs do (which I think is nearly the % offset by the 300 or so armour DKs have over us)

Deflection DR:
Paladin: BR (1,700-2,000) @ ~ 100% of attack taken
Death Knight: DS @ 6,000 healing / 5s best case scenario, worst case 6,000 / 10s (to activate Death Runes for HS spam), Rune Tap @ 7,000 / 30s

So Death Knights have very potent self healing potential coming in at approximately 800HTPS worst case to 1,400 HTPS best case. The healing is of course based on the DK rotation however you have some play in this and can offset by ~2s / choose to use Death Runes so it is potentially valuable.

Cool Downs:
Paladin: 60% DR @ 2 minutes
Death Knight: 37% DR @ 2 minutes

The main CD is pretty much equivalent, the Paladins win in terms of raw numbers for the DR however attacks are typically designed against the ~ 35%+ bonus swing (which rules out secondary CDs). For all intensive purposes this can be considered a wash imo other than for sustained damage scenarios.

Paladin: Heal to 30% on death @ 2 minutes
Death Knight: +15% health for 20s, +35% healing for 20s @ 2 minutes

Paladin cooldown covers the worst case scenario better (single shot, healer slacking), Death Knight allows for sustained damage phase of moderate increased damage and massively increased healing (including self healing to nearly 2,000HTPS for the duration using best case)

Paladin: 30% DR on damage under 35%
Death Knight: 15% DR on all damage for hits taking the DK under 35%, 15s ICD

Paladin version is superior for sustained damage, the DK version for single shot scenarios. From this it appears that really these are matched up wrongly. The GS effect is pretty equivalent to WoTN in the single shot scenario (100% to 0% giving the DK + 17.5% EH, the Paladin + 30%). The ICD / cooldown disparity would suggest that WoTN be weighted against our ADGS effect as a wash in favour of the DK in normal scenarios and the Paladin in worst case).

Paladin: 20% DR, -20% threat, 2 min CD
Death Knight: Absorb upto 50% health Magic Damage @75% for 6s @ 45s

Paladin: 100% heal @ 20 min CD
Death Knights: 45% magic DR @ Parry % (~ 20% giving +9% magic DR overall)

I am going to put these as a wash in terms of utility. The Paladin threat loss is crippling in fights where its important, and LoH can be used on others, at the same side the DK one is magic damage only so cannot be utilised on pure physical fights yet has a deflection like effect on magic fights that no other tank can match.

So basically in terms of CDs it basically seems to come down to AD (30% DR effect) vs Vampiric Blood. AD is probably the stronger of these two however the short CD on WoTN likely makes up for this in raw mitigation equivalence.

So from the looks of it, Blood DKs would appear to be on a similar level to Paladins at current with the EH differential not being able to account for the self healing / ICD on WoTN which basically can only be measured on average. This perhaps suggests that DK need a deflection mechanic.

This makes me think the best solution is something akin to:

Frost Presence: +5% base mitigation [+7%] (0.873 base multiplier, [0.854])
Blade Barrier: When both your Blood Runes are on CD triggers the blade barrier effect (15s duration), granting a % chance equal to your parry chance to reduce damage taken by [str*0.5], 3 charges.

In effect this mitigation should balance the base level of the tanks making them equal there with a deflection mechanic in place of mitigation on blade barrier which is similar in power to Holy Shield (~ 1,250 block). The lowered number of charges compared to HS reflects the ability to refresh faster (once per 5s under ideal conditions). This is not a full deflection mechanic, its not as powerful as Paladins full block (under ideal conditions it is lowered mitigation and uptime (due to non unhittable status) vs a 2.0s boss swing timer), nor as controllable as say Druid or Warrior versions which can maintain 100% uptime effectively.

That should balance out the raw damage taken numbers, the pure EH worst case I am not going to consider balancing given then unmeasurables in the scenario (self healing, potential, ICD on WoTN which reset on a very short timer compared to a proper CD). From this perspective I would say Tanks do not need to be balanced around raw EH provided Blizzard are not including one shot bosses that do not cover at least Tank Health * 1.5 ( > any secondary CD requiring 1 external CD minimum or 2 personal CDs). The issue arises with single shots at 1.01*-1.14*, or 1.3*-1.59* where by Paladins (and Druids due to higher EH) have a massive bonus due to the effective numbers on the cooldowns.

In terms of the other 2 DK specs more work needs to be done towards a stronger deflection talent to account for the lack of self healing. Likely both specs should have a high end + stamina talent to equalise base health making it easy to work out the danger zones of EH for the game designers.

Frost would likely aim for UA being something like salvation without the threat loss. Its mitigation technique being active in my mind:

Frozen Armour: 20s CD, 10s duration: Reduces physical damage taken by [str+500] for the duration.

This makes the damage taken spikier (given they are an avoidance build this makes sense) but high (like Shield Block for Warriors).

For Unholy I would simply increase the effectiveness of Blade Barrier:

Bone Blades: Your scourge strikes increase the number of charges on Blade Barrier by 1 and the damage absorbed by [str*0.25] for 5s, increases stamina by [3%]

Should work to equalise numbers a lot I think while still making DKs active tanks.
Any thoughts?

Blood DK vs Paladin seems like a fairly equal scenario bar 1 shot EH comparison where Paladins have an advantage. However this scenario covers such a small time period (0 heals within 2 boss swings) that it is very complex considering the interaction of controllable self healing and short ICD abilities.

Equalising overall base mitigation - removing the ~5s start of fight when a DK is vulnerable compared to other tanks:
Frost Presence: +5% base mitigation [+7%] (0.873 base multiplier, [0.854])

Adding a deflection equivalent making DKs damage taken roughly equivalent to Warriors (which is ~ equal to Paladins but spikier)
Blade Barrier: When both your Blood Runes are on CD triggers the blade barrier effect (15s duration), granting a % chance equal to your parry chance to reduce damage taken by [str*0.5], 3 charges.

Divine shield, a 5 minute CD 12s immunity with a 50% damage penalty while active, it is often seen in PvP to be overpowered given its effectiveness.

The Hunter equivalent is a 1 minute CD, 5s frontal 100% parry 100% spell deflection chance.

Seeing these two actually makes me believe that Divine Shield is underpowered for its cooldown. A sensible rework for this ability would be to place it in a similar role to deterence:

Divine Shield, 1 minute CD 5s immunity, cannot attack.

This would make it an active cooldown for usage and bring it in line with the other similar immunity talent. If you are wondering why it has to remain an immunity rather than an effective immunity the answer is the protection spec which would otherwise have the most awesome tanking cooldown ever (so the boss doesn't stop hitting me and I am immune... awesome).

I was reading through a blog (Ask in Game), and a few words sprang out at me:

"Epic quality gear is now trivial. It is so easy to obtain it has become a standard."

This is a very true statement, the availability of EPIC quality gear is problematic for the game, the availability of EPIC level gear is not problematic.

I see more and more pugs now asking to see the Epic achievement (to see how close you are, or indeed demanding it be complete). This measures two separate tasks, firstly that the loot is EPIC quality, and secondly that its iLevel is at least 213. What jumps out at me is that there are many good Superior quality items (JC Crab for instance) that are useful and yet their stat allowance is much lower than that of an Epic quality item.

Blizzard appear to have missed a trick here to top us ramping up stats so quickly, they can offer high iLevel lower quality items that have the same effect as offering low iLevel Epic items. Why is the gear that drops from a Heroic end boss Purple and Epic rather than simply a higher iLevel blue item?

if you think through it logically, why aren't the rewards the same across 10 and 25 man tiers, the iLevel (and thus scaling of vehicles) could be the same while offering us new qualities of armour:

Poor Quality - Vendor Trash
Normal Quality - White items
Uncommon Quality - Green Items - 5 Man
Superior Quality - Blue Items - 5 Man Heroic
Epic (10) Quality - Teal Items
Epic (25) Quality - Purple Items
Legendary Quality - Orange Items

It just seems that the iLevel of your gear should relate to your skill and experience, while its quality should relate to the difficulty in attaining it allowing us to actually differentiate more easily, if we want Ulduar geared people we ask for Teal (10) min Epic achievement, we can easily gauge instances by what we need from them rather than the hidden iLevel of doom.