Disclaimer - this is being written for the benefit of a new prot warrior in our guild. It is intended to serve as a guide to the basics. If you want to know more, then try Elitist Jerks (whose prot warrior guide is getting better as it gets longer and harder to read) or Tankspot. This is a draft version of the guide.
Your rating stats, in the order of best to worst are as follows:
Mastery > Parry > Dodge > Hit > Expertise
You should always reforge away hit or expertise - into mastery where possible. All items have two rating stats on them - the best items have mastery and parry, the worst have hit and expertise. Always look for mastery on your gear where you can.
Parry is better than dodge because of the Hold the Line talent.
Stamina gives you health, obviously. Agility gives you dodge, but you shouldn't gear for it or gem/enchant for it. It just turns up on gear sometimes. Strength is a threat stat, and again, don't ever gem/enchant for it.
Why is mastery so good?
Mastery increases block chance and crit block chance, but let's focus on block chance. The goal at present is for warriors to become "unhittable", that means every attack on the warrior either is avoided, or is blocked.
Since bosses are level 88 and we are level 85, they have an increased chance to hit us than you might expect from looking at your character sheet. In order for us to become unhittable our total avoidance needs to be 102.4%. That's quite a lot. A realistic initial goal is for us to become unhittable when you have shield block (+25% block chance) activated. That requires us to have total avoidance of 77.4%.
If you remember that there is a base 5% chance for the boss to simply miss with its attack, you need to have:
dodge chance + parry chance + block chance > 72.4%
Once you achieve that, or get very close to it, you will be amazed at the difference. Shield block is up for 1/4 of the time, and while it is up you will be like a piece of iron - your healers will love you. Essentially, shield block becomes almost as good as shield wall once you hit 72.4% avoidance.
Mastery is your best way of increasing avoidance - it's relatively cheap in terms of rating requirements, and it is not subject to diminishing returns. Stack it.
Why are hit and expertise so bad?
In dungeons and raids, you will get all the threat you need from the vengeance buff (which increases your damage the more damage you take). As a warrior in particular, vigilance will ensure that your vengeance reaches maximum very quickly in raids, and doesn't drop off very often.
In theory expertise is a better dps stat than hit, but since we have vengeance and don't need dps stats then hit's other benefit (making our interrupts more likely to land) makes hit more valuable. On fights where interrupts are critical some top players are building special hitcapped sets to make sure their interrupts never miss. You need 8% hit to do this but to begin with it may not be necessary.
Hit shield slam and revenge whenever they are up, and hit devastate if they aren't. Heroic strike when you have more than 50 or 60 rage. Don't forget about inner rage.
Concussive Blow and Shockwave do more damage than devastate and do lots more threat, so ideally you would have both of these in your rotation too. Personally however I prefer simple rotations, on many fights you'll want to save shockwave for adds, and anyway devastate is getting a big buff in 4.06 so the difference post patch will be pretty small I think.
There's lots to think about when tanking, so I save my precious brainpower for things other than a super-complicated six button rotation.
The cleverer among you will notice that I have not included rend in the above. I don't like the Blood and Thunder talent - I find it fiddly and unnecessary, and it requires me to have yet another button on my action bar. In heroics I maintain AoE aggro with glyphed cleave, revenge and thunderclap. It seems to work fine.
How to take less damage
The Thunderclap debuff and the demo shout debuff should be up on your target all the time. Practice this. They mitigate a huge amount of damage. Make sure that your UI shows those debuffs on your target, and maybe highlights them in some way or makes them bigger.
Make sure you are using shield block on cooldown.
Use your survival buttons - Enraged Regeneration, Last Stand and Shield Wall are all amazing, and are on short cooldowns. Don't save them for bosses! You can easily use one survival button on every trash pull, and be confident that there will be one available for the next pull.
Remember that Shield Wall is more effective if you press it BEFORE you take a big hit. If you have ALREADY taken a big hit and are low on health, use Last Stand instead.
Remember that Enraged Regen needs an enrage effect active on you to become available - press Berserker Rage to give you access to the regen.
Disarm almost never works on bosses, but normally works when a trash mob is hitting you with a sword or axe. It's a really solid damage reduction button.
Kiting! If you are taking lots of damage and near death why not stun the mobs using shockwave and heroic leap away? Or if you are fighting a boss, and the boss stops to cast some uninterruptable spell, why not run away? It will mean you take zero damage for 3 or 4 seconds while the mobs run back to you, giving your healers a bit of a break. Watch your threat while doing this though.
Spell reflect! I love tanking casters - zero damage taken, free rage and high dps.