Because I didn't want to create a new thread, I figured I might as well post it here, but I'm sort of beginning to relate to the OP's issue about taking a lot of damage. I've noticed that compared to healing Warriors and Paladins, Death Knights seem to drop like rocks when things turn sour. Being a Death Knight tank myself, I've experienced more than once that a healer would say "You're taking a lot of damage" (though never in the context of it being too much to handle).
My armory. (might not be working)
I'm working on my T8 helmet and then I'll be working towards new rings. I'm currently sitting at around 31.7k health in Frost Presence with 24k armor, 23% dodge and 15 or 17% parry, I don't remember precisely how much, unbuffed. Both dodge and parry goes up a little with Horn of Winter.
One thing I've found out is that standing with your back to a pack of mobs will take you down faster than you can type "o shi-". And sometimes positioning can be a ***** because mobs have this odd tendency to move around with you so on packs larger than three mobs you often end up with one behind you or next to you. This is arealy PITA in Culling of Stratholme and Gundrak (in my experience).
Considering we have an extra source of mitigation compared to Druids (parry) and much higher innate mitigation (with Improved Frost Presence you get 10% damage reduction, Blade Barrier is another 5%) than Paladins and Warriors, it really baffles me. Yet, I've seen it. Gundrak, for instance, was a shocking experience with my healer because I saw the T8/T9 geared Death Knight tank drop from 100% health (with HoTs on) to 30% in less than three seconds on a pack of trash (well, I think there were two packs to be fair, but still).
A Warrior could've popped Shield Block and reduced all incoming damage (except poisons and magic) by 100% or close for 10 seconds. Paladins could've used Hand of Protection, though I'm not sure if it wiped aggro or not. Maybe the Death Knight was just poor at damage mitigation, because I didn't see him pop Icebound Fortitude or Vampiric Blood.
In any case, when I'm tanking on my Death Knight I feel more like a meat shield than anything else. My Warrior and Paladin both hide behind a giant shield, giving the impression that they are in fact a block of steel. The Druid hides in a different shape, but the Death Knight looks just like the DPS Death Knight next to him. I remember thinking Death Knight tanking was awesome, because I could sacrifice a bit of personal DPS, tank the mobs and still do enough damage to balance out the equation. I still do a lot of damage while tanking (1600-2000 dps on a HC Nexus run), but I don't feel like I'm the tank. I feel more like I'm the guy who pulled aggro and never stopped.
*shrug*