Use: Permanently teaches you this glyph. Increases the radius of Whirlwind by 4 yards.
Sell Price: 1
Although this causes no extra threat specifically, doing normal weapon damage to four enemies at once can go a long way to keeping aggro in a cluster of mobs. Also, each individual hit has the normal chance of going critical. Two or more criticals from a large two-hander can make a decisive difference to the average cluster of mobs.
The rage cost is a little on the steep side, so this is best used in conjunction with [Berserker Rage] to compensate. Note that unlike most warrior abilities, this one is truly AoE and will hit targets directly behind the warrior.
When using two one-handed weapons, it hits with both weapons.
Be careful of crowd control with whirlwind however. You shouldn't be breaking your teammates sheep/shackle/seduce.
This attack also has an additional use in PvP - due to the fact it can hit anyone in melee range regardless of where they are attacking you from, it can be used to teach a lesson to that irritating Rogue who is constantly backstabbing you.
With its 8 yard range, it can hit enemies in your dead zone (Allows you to hit enemies too close for you to intercept / charge, yet still far enough that if they're out of your melee range, you can hit them).
Once you get [Titan's Grip] (Fury spec), Whirlwind causes very high damage, making it worth fitting in all your rotations, even single target. After all, causing instant damage with double two-handers is always good.
Patch 5.2.0 (5-Mar-2013): Whirlwind now costs 20 rage (was 30) for Arms Warriors. The cost is unchanged for Fury Warriors.
Patch 4.1.0 (26-Apr-2011): Whirlwind now has its cooldown reduced by 6 seconds when it deals damage to 4 or more targets. The Whirlwind effect caused by Bladestorm remains unchanged.
Patch 2.4.2 (2008-05-13): Whirlwind now has the correct sound associated with it.
Patch 2.4.0 (25-Mar-2008): Critical strikes with the off-hand weapon from this ability can now trigger Flurry and Rampage.
Patch 2.3.0 (13-Nov-2007): This ability now strikes with both weapons when a Warrior is dual-wielding.
Patch 1.8.0 (10-Oct-2005): Now uses normalized attack power.