We've known for a while that Cataclysm was going to feature some extensive changes to the stat and gear systems, and while we'd gotten bits and pieces of the whole picture before, the blues recently posted a massive breakdown of many of the changes. Death knight tanks will see a few things affect them as well, such as the complete removal of defense and a change to the workings of parry. Without further ado, let's break down the major changes that affect death knight tanking.
Stamina: The gap between plate-wears and non-plate wearers for stamina will shrink, as stat reallocation leads to more stamina on armor across the board. This probably won't have too much of a massive effect on us, since we'll still come out on top with Frost Presence, talents, and gems. It's also worth noting that we may actually see less stamina on some tank gear than some DPS gear, as we'll be expected to make up the balance with stamina gems.
Armor: Due to re-balancing, there will be less bonus armor, and other armor classes will be a lot closer to plate than they are now. Again, we probably won't see too much of a change here. Our talents will make up the difference, so I doubt you'll see shamans stealing our tanking slots just because they have more armor.
Parry: Parry will no longer speed up attacks. That's right, parry gibbing is a thing of the past. This means it should be easier to try out frost dual wield tanking, at the least. In addition, parry will no longer 100% avoid attacks, but will instead reduce the damage of two attacks by 50%. The overall effect should be near the same, though this may perhaps to be an attempt at making health loss less spiky for tanks. Note, however, that dodge is still unchanged.
Mastery: Mastery is still a bit of a wild card for death knight tanks. We know mastery will appear on gear, and is supposed to provide bonuses to key stats based on your class and spec. However, since death knights can spec into either DPS or tanking in a single tree, we can't yet say for sure which stats Mastery will provide, or how the game will determine whether to grant us tank mastery stats or DPS mastery stats. The devs have promised further words on how they will shake down soon, though.
Resilience: Resilience will now protect against critical strikes from other players only, so don't expect to use it to tank. Luckily, you won't need critical strike avoidance to tank anyway. See the next bit of news for more on that.
Defense: Defense will also be going away. We'll be getting automatic critical strike immunity from Frost Presence by itself. It's a bit late for those of us who were struggling to find 540 defense so we could tank heroics back in the day, but still welcome. This should also be a major step toward Blizzard's stated objective of allowing DPS-specced tank classes to tank a quick 5-man dungeon if needed. Defense on gear will be replaced by dodge, parry, or block rating. Hopefully, they keep the block rating primarily on paladin and warrior tier gear so we aren't stuck with stats we can't use.
Attack Power: Attack Power will be going away on gear, to be replaced by agility or strength. Now, normally, this isn't a problem for us. Tank gear almost universally has strength on it, so we won't see much of a change there. However, this may come up as an issue for tanks using polearms. Polearms often find their way into the hands of death knight tanks. They're perfectly serviceable weapons, and the agility present on most of them make them arguably superior tanking weapons. With attack power being removed from them, however, polearm users will see a bit of a drop in threat. On the plus side, they should also see a rise in stamina and agility on the weapons, which may very well make up for it.
World of Warcraft: Cataclysm will destroy Azeroth as we know it. Nothing will be the same. In WoW.com's Guide to Cataclysm you can find out everything you need to know about WoW's third expansion. From Goblins and Worgens to Mastery and Guild changes, it's all there for your cataclysmic enjoyment.