Well, any melee DPS can tell you that a dodged attack doesn't do any damage. For warriors, who generate the rage we use to use our special abilities by dealing and taking damage, not doing damage is bad. Furthermore, except for the various splash or AoE damage abilities bosses have, a DPSing warrior doesn't get a lot of rage from damage taken, and has to rely almost exclusively on the white damage he or she deals to generate rage. So annoying things like missing and being dodged? They leave you stranded in rageless limbo, desperately creeping your finger towards the slam button, hoping for rage so you can dump a few .5 second slams into Vashj's spinal column. Not that I know what that feels like. It takes about 26 expertise, or 106 expertise rating, to avoid dodge chance completely assuming that dodge chance is at 5%, so while there's relatively little plate out there with expertise on it, a DPS warrior could keep his or her eye out for the leather gear that has it and get up to the cap fairly easily. Just let the rogues (and possibly enhancement shamans) get it first or they'll stab you in the back when you aren't looking, and with their high expertise rating you won't be able to dodge.
As nice as expertise is for a melee DPS (be it a rogue, a warrior, an enhancement shaman, a feral druid or a retribution paladin) it has an even greater benefit for tanking. Expertise is basically both a threat and an avoidance stat for tanks. Along with + hit, expertise helps with both rage generation (you get rage when you deal damage, although that's admittedly not usually all that much of an issue for a tank who is taking a beating and thus getting rage anyway) and threat generation, since a dodged or parried attack doesn't land, and thus doesn't generate any threat. These are both fine things, but expertise also has another, hidden benefit. By reducing parries, expertise reduces the chance a boss will get a hasted parry counterattack. This isn't just your own parries, of course, and it's a major reason why melee DPS must get behind the boss: attacks from behind cannot be parried, and so, smart melee DPS lined up behind a boss will not give the boss a lot of parries which he or she will use to annihilate any and all traces of the tank from existence in a spray of viscera. Forget any and all uncrushability talents your warrior or paladin might have, if the boss gets a string of parries because the melee are next to the tank in parry range, your tank is going to die.
Proper positioning for the melee allows your tank to avoid that danger. Expertise allows your tank to further minimize that chance by reducing the chance for his or her attacks to be parried as well, thus keeping Karathress (for example) from crushing your face in with that big spiky mace of his. It's uncertain how much parry chance bosses have: it was originally assumed that they had roughly 5% to dodge and parry alike, but more recently I've seen the parry chance estimated at between 12 and 15%. Elitist Jerks pegs it at about 10%, but I tend to be more pessimistic and go for the 15% mark. What does this mean?
For an orc warrior with full defiance and the following gear (Gauntlets of Enforcement, Girdle of the Fearless, Sunguard Legplates, The Brutalizer, Bracers of the Ancient Phalanx, the Brooch of Deftness, Band of Determination, and the Shapeshifter's Signet) would have157 expertise rating from gear and a total of 53 expertise, if my trusty calculator is to be believed. (It taking 3.9 points of expertise rating to gain 1 point of expertise, then adding in the orc racial and the derfiance talent boost.) This would reduce your chance to be parried by roughly 13.25%, again trusting in my calculator. (Please, if you have better math skills than me, please offer them up.) So if bosses parry at roughly 10%, as the Elitist Jerks post suggests, then it is possible for an orc to wear slightly less than this level of expertise gear and be 'capped', which is to say he can completely remove the chance for a boss to parry him. However, if the chance is 15%, he will still be parried some of the time. A human warrior with access to this level of gear would go for the Dragonscale Encrusted Longblade or Mallet of the Tides instead, skewing the numbers somewhat. We see that it's possible to get a great deal of expertise rating for a tanking warrior, but that depending on how high or low the boss' parry is, you may or may not be able to 'cap' the ability, and of course you never want to cap expertise if it's going to cost you too much in terms of stamina, armor, defense and so on. (In my estimation it should still be possible with careful gear selection for a warrior to equip everything in the list above and still be uncrittable. I'm not saying you'd want to, I'm just saying it can be done.)
Whatever the chance to parry ultimately is, expertise is still a valuable stat. Even reducing the chance to be dodged or parried by 1% is valuable to a tank. I'd definitely advise building an 'expertise set' to swap in on fights with a lot of burst physical damage or who require a lot of threat generation up front. Unfortunately, it's not terribly easy for a new tank to accumulate a lot of expertise gear yet: the new badge loot released in patch 2.4 will help to some degree, and there's still the good ol' Brooch of Deftness available for cheap. Here's a list of all the expertise gear new to the Burning Crusade (as opposed to old gear changed to have expertise rating) to get you started.
Next week, we'll talk about something else. I mean, it's not like I ever shut up. But for this week, if you have better math or insight into how expertise works, please contribute to the comments, it's what they're there for. I admit I'm still working on my expertise set myself. (I'm so going to get the Girdle of the Fearless the second the anvil drops on Norgannon.)