Defensive Stance is a cautious back-footed stance that allows the warrior to react more efficiently to incoming attacks, and better utilize the sheer immensity of armor available to him. The slower, more deliberate nature of the stance makes it difficult to inflict as much damage with his own attacks. This style of fighting lends itself to shield use with main-handed weapons for the most effect, but can be successfully used with any type of weapon to a lesser degree.
A must for main tank work, the 10% incoming damage reduction, increased threat, and access to Taunt make all the difference when working in a group. Since several of the Defensive Stance abilities require one, a shield is a must to get the most out of this stance.
This stance does have some place in solo work, as on average, most warriors will be doing a lot more than 5% extra damage than a similarly leveled non-elite mob per swing, meaning that if a fight looks like it could go either way, a stance swap to Defensive will more often than not allow the warrior to survive long enough to finish the job, particularly if the mob has status effects built up already — Sunder Armor, Rend and so on.
Under normal circumstances, however, it is an unnecessarily slow way to go one-on-one vs solo mobs, and Battle Stance or Berserker Stance would achieve the same effect in a shorter time, with minimal extra risk.
In PvP, this stance is less helpful because combat is generally decided by who can cause the most critical hits and stun or fear effects in as short a time as possible. Survivability usually comes second to "Shock and Awe". Use Defensive Stance only briefly to allow use of particular abilities; Disarm, Shield Wall, or if you wish to reduce damage that you know will come soon by 10%/20%.
Quests
This ability is gained through completing one of several quests: