Stamina In Wrath, stamina was more than king -- it was grand high emperor. Despite the fact that Wrath supposedly removed crushing blows, boss fights were still almost all built around the idea of each boss dealing out huge amounts of damage in a short time, requiring tanks to stack as much stamina as possible in order to survive these hits. In Cataclysm, bosses are a little more forgiving on how much damage they do, at least so far.
This doesn't mean stamina is unimportant, however. It's probably still the most important tank stat. However, there will in theory be a cutoff point at which your healer will be able to heal you from a few hits, and you'd be better off stacking mastery or avoidance to keep the damage from happening at all. That said, no one's quite sure where this cutoff point is yet, and it may actually be different for each raid group or even each boss.
Regardless, most people at least believe you probably don't need to focus on gemming stamina as much anymore. When you have 150k HP, the extra 5k you get from gemming just isn't as important as it used to be. At the least, you can feel free to use some purple and green gems where needed to get more avoidance, mastery, hit rating, or expertise.
Mastery At the opposite end of the spectrum from unholy's dismal (for now) form of mastery, blood's mastery, Blood Shield, is incredibly good. It's essentially our own version of shield block, a focused burst of physical damage absorption. It's better than avoidance in that Blood Shield can absorb damage that cannot be parried or dodged. On the other hand, it's worse than avoidance in that it must be activated. You must hit the Death Strike button, and you must activate it often.
If you choose to stack mastery, you should probably be eschewing the use of Heart Strike as much as possible to churn out more Death Strikes. Just be careful, as new Blood Shields will overwrite old ones, so optimally, you want to make sure your last Blood Shield is gone before putting up a new one. In theory, this puts a soft cap or diminishing returns on mastery at various points if you are consistently overwriting an old Blood Shield with a new one. Also, if you decide to go with mastery stacking, you will probably want to make sure you also make a good run at getting close to the 8% hit soft cap and the 26 expertise soft cap, as missing a Death Strike means you don't get a Blood Shield, which means you're about to be in a world of hurt.
Avoidance Dodge and parry rating are your next defense options. There are two big advantages for avoidance. First, it's innate. You don't have to hit with a Death Strike to avoid an attack. Second, it avoids all damage from a hit. If you dodge or parry an attack, you take no damage. At the same time, it's also random. You can't control which hit you avoid. This means if you have a bad run, you could suddenly be taking a lot of extra damage from a bad string of high hits.
Still, a lot of people argue that a good, solid avoidance stat is just as good in the long run as mastery, as long as your healer doesn't mind the damage being a little more spiky. When gearing or gemming for avoidance, your goal should always be to raise the stat that's lower out of parry or dodge. This is because both stats have equal diminishing returns now, so the lower stat will always give a bit more bang for your buck.
So what's a tank to do? People are still deciding, and in fact, a lot of people are seeing various amounts of success even in heroic raiding by using various styles. At this point, it's mostly anecdotal, but since most of the math is in the preliminary stages and it's not even clear if some of this stuff can be properly mathed, it's the best we have.
One of the more popular methods is to choose to get either mastery or avoidance to a certain comfortable level or soft cap (such as 25-30% total avoidance, or 90-100% conversion from mastery), then gem, reforge, and gear for the other stats. This allows you a nice balance that'll give you some control over your damage intake via sizable blood shields, while not completely neglecting that avoidance can sometimes be superior in giving you a quick breather.
I have to admit that for the moment, I myself am biased toward stacking mastery. I always like feeling like I'm the master of my own destiny, so being able to control my damage intake in a very real way with some well-timed Blood Shields feels both skillful and high control. Whether or not that pans out in the long run is a little bit murky. The math is still being compiled, and depending on who you ask, you may get a different answer. In fact, it may be we won't have a clear winner until tier 12 or 13, when we can stack even higher levels of mastery or avoidance and see where they take us.
Which stat reigns supreme?
If you'd like to get a little more into the math and theory side of this debate, I'd recommend checking out the pwnwear forums, especially this thread, which dates back to October and follows the evolution of the debate from then until now, complete with enough math to give a liberal arts major a splitting headache. As more guilds get into the raiding side of things and we get more data from more death knight tanks, it may be that we'll get enough of a sample size to establish once and for all which tanking stat reigns supreme. Of course, if we don't, maybe that isn't such a bad thing after all.
Follow our road map to leveling your DK from 80 to 85, and then learn the ropes of endgame play with WoW Insider's DK 101 guide. Gear up with pre-heroic gear for DPS DKs and keep track of what's happening on your hotbars with our guide to DK spell alerts.