Slam the target with your shield, causing X to Y damage (based on attack power) and dispelling 1 magic effect on the target.
Shield Slam is a massive direct damage attack which dispels one magic effect. The dispel effect is useful against a large amount of mobs and bosses while also allowing you to take things like Power Word: Shield or Power Word: Fortitude off a player, which is also useful in PvP. The ability offers a good solid chunk of (potentially critical) damage, which is independent of equipped weapon stats, and can even be delivered while disarmed, if need be.
Patch 4.0.3 (15-Nov-2010): Contribution of attack power reduced to 60%, down from 75%, and base damage brought up so that a level 85 warrior in Heroic dungeon gear deals the same damage with no Vengeance, but Vengeance has less impact. In addition, Shield Slam no longer generates 30% bonus threat from its damage.
Patch 3.3.2 (2010-01-02): The damage scaling from block value for this ability now diminishes faster and diminishes starting at a lower block value. The difference should be negligible for players in high-end tanking armor. In addition, the threat caused by Shield Slam has been increased by 30%.
Patch 3.2.0 (04-Aug-2009): The benefit from additional block value this ability gains is now subject to diminishing returns. Diminishing returns occur once block value exceeds 30 times the player's level and caps the maximum damage benefit from shield block value at 37.5 times the player's level.
/Patch 3.0.2 (14-Oct-2008): Shield Slam is now available to all warriors, starting at level 40 and damage slightly increased.
Patch 2.3.0 (13-Nov-2007): Shield Slam now always tries to dispel one Magic effect on the target.