Retaliation is a self-buff that costs no Rage. It has a 5 minute cooldown. As of patch 3.0.2, it is NO longer linked to the Shield Wall or the Recklessness timers. While it is active, the first 20 melee attacks taken from the front will result in an instant counterattack with your weapon. It has a very obvious animation making its usefulness in PvP hinge on the opponents knowledge of the ability as many opponents will simply stop attacking the warrior when they see the spinning blades—this however could be viewed as a defensive advantage. Available in Battle Stance only.
Before the implementation of its animation this was one of your most powerful abilities, now it is at best of marginal utility in PvE. The counterattacks from Retaliation are not instant attacks, they are normal swings. Because of this, Retaliation is most powerful when used with an extremely slow two-handed weapon.
Although it has a cap on how many counterattacks you can make, it is higher than your average mob will trigger in a fight. So, this skill is best used against multiple enemies, particularly if they attack quickly.
Warrior-Tanks can greatly benefit from Retaliation's multiple target threat by quickly switching to Battle Stance, activating Retaliation and then returning to Defensive Stance. As stance changes remove nearly all built up Rage this is best done when the warrior is already low on Rage.