Protection guide

Warrior Tank.

Rotation: There is no rotation when playing Protection Warrior. It is played with a priority system, we have to be proactive in using all our abilities to avoid damage taken to the group and ourselves at the right time.

Stats: Mastery is the best secondary stat, it increases our block chance and the excess is converted into critical block chance if our block chance exceeds 100%. So the more mastery you have, the better. Parry is better than dodge due to Hold The Line talent, but the more mobs hitting you the more important it is to have an even parry : dodge ratio, since the more often you are being hit, the higher uptime you will have on Hold The Line. We also don't want to negate dodge completely since if we only stacked parry (or dodge) our avoidance would be much lower due to diminishing returns on both parry and dodge.

Single target threat: Shield Slam generates the most threat, it is also good to open with a Shield Block -> Shield Slam for even more threat through the Heavy Repercussions talent. After Shield Slam, Revenge deals highest threat and hits two targets when glyphed. When you don't have Shield Slam or Revenge, use Devastate, and applying 3 stacks of sunder armor is important especially if you have physical damage classes in the group. However, Rend provides more threat than devastate when when the target has the 30% bleed debuff from a Blood Frenzy Arms Warrior, Hemorrhage Subtelty Rogue (rare) or Mangle Feral Druid, after 4 ticks. 6 ticks without the debuff. You may proc a free Shield Slam with Sword And Board by using either Devastate or Revenge. If you are generating a lot of rage and need to build threat, hit Inner Rage and use War Academy + Incite buffed Heroic Strike every time it is off cooldown in combination with your GCD abilities.

Aoe threat: Rend a mob and then Thunderclap to afflict surrounding mobs with Blood And Thunder. Then use Inner Rage > Cleaves with Revenge and Devastate, and use Thunder Clap every time it is ready. Use Shockwave whenever you want, but you can make more damage and threat with it after 2 or 3 stacks of Thunder Struck.

Defense and bosses: Have Vigilance on the other tank or a high threat player. Use and refresh Demoralizing Shout and Thunder Clap to reduce the incoming damage and rotate Shield Block everytime it is off cooldown. Use Shield Wall in emergencies or to reduce an incoming damage spike. Use Last Stand / Rallying Cry in emergencies. Intervene for mobility and to help a player taking damage.

Enchants / gemming / glyphs / talents

Enchant your weapon with Windwalk or Mending, take Protection on cloak and Greater Stamina on chest. Take Mastery on gloves and boots.
Gem for mastery (yellow) mastery + stamina (green) and mastery + parry (orange).
Prime: Revenge, Devastate, Shield Slam
Major: Shockwave, Cleaving
Minor: Berserker Rage, Demoralising Shout
Recommended: Take Long Charge or Sunder Armor as the third major glyph, don't take thunderclap as it can be a nuicance around cc. Take Intimidating Shout as the 3rd minor glyph, it's a great trash cc / interrupt. Glyphed Berserker Rage helps maximise your rage, especially useful when making use of Inner Rage.
Talents: Very customizable. I like to take 5 from Arms, War Academy and Field Dressing. 5 from Fury, Blood Craze and Cruelty, and in the Protection tree, leave out Gag Order, Impending Victory and Safeguard, only taking 1 point in Shield Specialization. I like Safeguard but Gag Order only has a use vs caster trash and Impending Victory isn't very effective imo. Take 1 point from War Academy and you can grab Piercing Howl, a great utility in aoe, but at the cost of a slight loss in Devastate threat generation and damage. For better damage in aoe, the full 3 points in Shield Specialization will allow you, with Shield Block active, to rotate Cleave with higher uptime.

Further reading: http://elitistjerks.com/f81/t110315-cataclysm_protection_warrior/
http://elitistjerks.com/f81/t110350-cataclysm_warrior_faq_4_2_read_while_patching_before_posting/