Tanking is two synergistic activities rolled into one, it's the hockey of WoW. Much as hockey combines skilled skating and beating the crap out of people (I understand there's a small black object that's important in there somewhere too) so does tanking involve annoying things while surviving the results of their annoyance. It's easy as a tank to fall too far into the 'keep yourself alive' mentality by focusing all of your gearing attention on stamina, avoidance and mitigation. This will make you a much easier to heal tank, and I'm sure your healers would thank you for it if not for the horde of monsters that walked over and ripped their faces off because you didn't build any threat.
Even in the new age where there's more baseline threat in defensive stance and warriors generally output more threat via damage, with new abilities like Damage Shield and Shockwave, you still have to pay attention to the stats that help you generate threat as well as the ones that keep you alive. We'll look at the Daunting Legplates as an example of a reasonably high stamina armor piece with very high defense, but the only threat statistic they have is the 54 strength. (Strength for a warrior tank is both a threat and mitigation statisic due to its relation to how much damage a warrior blocks - 2 strength is 1 shield block value) They aren't bad legs and no one is saying they are, they're simply focused more purely on stamina and defense in order to allow tanks just starting out to try and reach 535 defense rating and become uncrittable to mobs in heroic dungeons at level 80.
Now we'll compare them with the Ley-Guardian's Legguards, from heroic Oculus, the kind of instance you'd get the Daunting to be able to run. The Ley-Guardian's have more stamina, slightly less strength, a socket (yellow, not the first choice for tanks but allows for customization), almost as much defense as the Daunting and a large chunk of hit rating. That hit rating by itself makes these legs superior to the Daunting for threat generation, for two reasons. The first is that an attack that doesn't hit the mob doesn't generate any threat. The second is that an attack that doesn't hit the mob doesn't generate any rage, which you as a warrior tank use to generate threat via specials. Hit on your gear also affects whether or not your taunts are successful. This does not mean you have to stack hit to the exclusion of all other stats: in general, once you have 5% hit (163 hit rating) you probably have all the hit you need, since as a warrior tank you're using a one handed weapon (or a two handed weapon in your main hand which no longer suffers an additional hit penalty if you're one of those crazy Titan's Grip warriors who still likes to tank) and a shield. If as a tank you wanted to completely guarantee that your specials would always hit (including Devastate, Shield Slam, Shockwave, Thunder Clap, Revenge and Heroic Strike) then you could gear up to 8% hit (262 hit rating), but it's not strictly speaking necessary and very hard to do before 25 man raiding gear.
Expertise serves a similar function, save that rather than removing misses it pushes dodges and parries off of the attack table. Not only do dodged or parried attacks have the same effect as misses for a tank (you don't generate threat on a dodged/parried attack and you don't get rage for it either) but in addition to this, a parried attack from a boss causes his next attack to land faster, increasing the damage the tank takes. This means that expertise is both a threat and an avoidance stat for warrior tanks much as strength is a threat and mitigation stat.
Now, with the changes to crushing blows, level 80 tanks will never see a true 'parry gib' again. Parry gibbing in BC was when a boss (at effectively level 83, skull level mobs like raid bosses) would parry the tank or someone else in the case of a poorly positioned melee DPS and then get a hasted counterattack on the tank which would be a crushing blow, resulting in a spike of damage that required a very quick response from a healer. Since it couldn't really be predicted, it often resulted in tanks going from full health to dead before the healers could react, which made expertise a very important stat indeed for tanks.
Each class had their ways to push crushing blows off of the attack table or survive them, but those means weren't as effective (especially the warrior method of spamming shield block) when an attack was parried. In Wrath of the Lich King, this isn't possible for level 80 warrior tanks because crushing blows are only possible if the mob is four levels above the player, which means skull level boss mobs don't crush in raids anymore. Still, while expertise will no longer prevent a true parry gibbing since those crushing blows are gone, it still does reduce the incoming damage to the tank, making it a stat that performs double duty for a tanking warrior.
So how much expertise do you need? You can keep stacking expertise (and keep seeing a benefit from it) until you've completely removed the chance to be parried, but at level 80 it costs 32.79 expertise rating to reduce chance to be dodged or parried by 1%, and I've seen estimates of boss parry rates at up to 12 to 15%. If we assume 12% is correct, you would need 394 expertise rating to completely reduce parries, and I don't think that's possible at this time. (I haven't combed the entire loot tables of all 10 and 25 man raids for expertise gear, though, so it might be possible, but I don't think it would be advisable.) Furthermore, that kind of relentless dedication to expertise rating over all other stats would probably hurt you in too many other areas like stamina, hit, SBV, dodge and so on to be worth it.
I would say gearing to remove the 5.75% chance an attack will be dodged by a boss is more than sufficient, which would be 189 expertise rating (rounded up), or almost 24 expertise totally unmodified by talents or racial abilities. (I think it would actually take 23 expertise to reduce dodge chance by 5.75% and 24 to reduce it by 6%.) This will probably require significant investment by most warrior tanks but luckily the talent Vitality can add up to 6 expertise, meaning that you'd only need 18 expertise. That is fairly easily achieveable. However, it needs to be said that more expertise as a tank is never going to be a bad thing unless it's achieved at the cost of all else.
Again, please don't take this to mean that you should go for hit and expertise over all else. We've talked before about the dangers of overstacking stats. It's very important to try and keep a well rounded approach and probably you'll need two or three different sets of tanking gear if you're looking to tank primarily.
Next week, we'll cover how hit and expertise function for DPS warriors, both arms and fury.