Spamming Death Strike works
In theory, Death Strike would be a great reactive ability. Getting low on health? Death Strike. Just took a huge hit? Death Strike, because you know that means you'll get a huge heal and thus a huge Blood Shield from the heal.
Unfortunately, in practice, all you really end up doing is spamming Death Strike. This is essentially because Death Strike and Blood Shield replace block, which shield classes get automatically. Without thinking, they just don't take as much damage at the base, because their shield automatically blocks it more often than not, especially once they get to decent levels of mastery.
As an example, consider a death knight and a warrior, each tanking the same raid boss. Let's say they have similar skill and gear levels, and they each take a huge spike of damage from the boss. Now, the warrior (thanks to high mastery) is going to end that streak of damage at 40% health because he had block. The death knight, in the meantime, is at 10% health because he can't block and is expected to make up that health gap by immediately throwing out a few Death Strikes.
Now, all well and good you say. Just spam Death Strike, and you're fine. In theory, this is true. The issue, though, is that in spamming Death Strike, you've immediately used 4 runes. That means 4 runes that can't be used for at least 8 seconds. Those runes could have been used to apply diseases or throw up Bone Shield or Death and Decay (or even, say, a Blood Boil or Heart Strike, if you were in death rune mode). Those resources are gone.
On the other hand, if you decide to use those runes to, say, refresh Frost Fever and Blood Plague, that's a whole mess of healing and blocking power you're giving up, and your healer either needs to pour extra mana on you or gamble that you won't get hit quite hard enough to die while you wait for those runes to refresh for another Death Strike.
In the meantime, the warrior is now free to throw up whatever threat-gaining abilities, defensive cooldowns, or debuffs he wants without worrying about wasting resources that should have been used to pull himself back from the brink of death. He will be, by definition, ahead of us.
Runic Empowerment: Surprisingly disempowering
Of course, even spamming Death Strike becomes a whole game in itself for a simple reason: Our rune system works against us when we try to get the maximum amount of Death Strikes out.
Most of this is due to Runic Empowerment. This ability regenerates runes at random when you use your runic power dumps. On the face of things, this is great. If you're lucky, you get a quick streak of runes up, and you can immediately use theme for another Death Strike, building your blood shield up and avoiding more damage.
Unfortunately, the wrench in the works here are your blood runes. They simply can't be used for Death Strikes, Blood Tap aside. Now, that might encourage you to leave them unspent; just let them pool so any Runic Empowerment procs go straight to your frost and unholy runes. That doesn't work well either, though, because you do want your blood runes spent for Blade Barrier.
Thus, what you end up doing is staring at your rune bar, trying to balance all your runes for optimum generation of the frost and unholy runes you need for Death Strike. This generally means you have 1 blood rune spent at all times, spending the active blood rune only when the second blood rune is about to refresh. This allows you to keep Blade Barrier up as much as possible, while still keeping blood runes up as much as possible so the minimum amount of Runic Empowerment procs get wasted on them. Then, you just have to hope that any Runic Empowerment procs give you the right combination of runes to get off another Death Strike -- the last thing you want is a combination of 2 unholy or 2 frost runes.
In addition, you need to watch your runic power. If it's capped and you're still trying to get your blood runes balanced so you don't proc Runic Empowerment on them, you're probably losing threat and Runic Empowerment procs anyway. This whole system is popularly called "rune Tetris" and is considered the #1 issue with blood tanks by many people, just because of how much more involved and stressful it is to manage.
It is interesting to note that, these days, blood is the only tree that has to manage runes like this. Frost now has permanent death runes, which means just about any rune that refreshes for them will automatically be useful for their main strike, Obliterate. Unholy can spec into Runic Corruption, which just offers a quick boost to rune refresh rate in general. Again, no worries about getting the correct rune there, because they're all correct.
The most obvious answer here, then, is to give blood death knights one of these workarounds. Making Runic Corruption baseline would go a long way toward ending rune Tetris and allowing death knights to glance up from their rune bars every once in a while.
Fighting against themselves
Even beyond rune Tetris, there are other aspects of death knight tanking that are clunky enough to make playing the class frustrating in ways that just aren't true of other tanking specs. In short, a lot of our defensive abilities cancel each other out or at least are constantly vying for dominance.
At the most basic level, you can argue that avoidance (and thus Bone Shield) work at cross purposes with Blood Shield. Since your Blood Shield is only as powerful as how much damage you've taken, any avoidance of that damage weakens Blood Shield. That's sort of a low-level annoyance, arguably, but it's there.
The biggest issue, though, is the application of diseases and the debuffs they cause. The damage diseases cause is actually pretty solid threat, but where they really shine is debuffs. The reduced physical damage from Scarlet Fever and the reduced speed from Frost Fever are both incredibly powerful tools for making you take damage. The problem is that casting them when Outbreak is on cooldown requires a frost and unholy rune, which means you lose a Death Strike. This in itself can be lethal in the short term, even if those diseases will protect you from damage in the long term. In addition, spreading them to multiple targets requires the use of Pestilence, which could unbalance rune Tetris when used and, regardless, doesn't generate any extra threat. Compare this to, say, Thunder Clap, which does AOE threat at the same time it spreads both extra damage and the slowing debuff.
Blizzard CM Zarhym has already discussed the possibility of solving this by giving blood a reduced cooldown on Outbreak. This would at least give DKs a chance to refresh diseases without giving up a Death Strike. Another possibility is to give Pestilence or Blood Boil the ability to refresh all diseases. This wouldn't specifically solve the rune Tetris issue, but it would at least allow blood death knights to use all their defensive tools to the something approaching the same level that other tank classes can.
Mastering mastery may merit mastery modification
Of course, there's also another, more nuclear option to be had. Just give death knights a different mastery, one that isn't so focused on using a single attack as much as possible. One solution is to possibly give us the ability to proc a Blood Shield from multiple attacks, similar to the druid's Savage Defense. Of course, if you talk to many druid tanks, they'll tell you Savage Defense has problems of its own. Perhaps then, there could be a move to return death knights more to the unholy avoidance tank model from the Wrath era. Turn Bone Shield into a sort of passive mastery ability that procs more charges every time an attack is successfully avoided and/or absorbs damage based on previous damage taken just as Blood Shield does now.
There are a lot of ideas to fixing mastery and a lot of ways to go about it. There are still some big roadblocks, though. Since death knights, even with their clunky playstyle, are still more than adequate tanks up through heroic dungeons and even in normal raiding, a lot of players don't even realize they have issues. As long as they're surviving and downing content, players don't care that the player behind the blood death knight tank is working twice as hard as the warrior tank.
In addition, Blizzard has said in the past that it likes the "active mitigation" aspect of the death knight tanking model and would, in fact, prefer that all tanking classes have it. Now, many members of the death knight tanking community have pointed out that it's not really active if the only optimal way of playing is to spam the active button. It's a tossup as to whether Blizzard will buy that argument, but we have seen some blue posts in death knight tanking threads, so we can at least hope that Blizzard is aware of these arguments.
The real test, of course, will be when the theoretical patch 4.2.1 rolls around. Will we see some meaningful death knight tanking changes, or will we need to keep queuing up the Russian folk music remixes?
Learn the ropes of endgame play with WoW Insider's DK 101 guide. Make yourself invaluable to your raid group with Mind Freeze and other interrupts, gear up with pre-heroic DPS gear or pre-heroic tank gear, and plot your path to tier 11/valor point DPS gear.