Mocking blow is a Warrior ability that deals weapon damage and forces a target to attack you for a brief period of time. If you have problems Taunting an enemy off a party member, or don't have time to switch to defensive stance, this may be something useful.
Mocking Blow is a 10 Rage ability that deals some damage and forces a target to attack you for six seconds. Unfortunately, it seems that after the six seconds, the mob resumes attacking whatever it was attacking previously. This is pretty much just an emergency ability to pull someone off a party member before switching to defensive stance and Taunting it. Unlike Taunt and Challenging Shout this ability can be used while silenced.
This is basically a powerful emergency taunt. This can be a very effective tactic if your taunt is resisted.
In PvP it's not very useful, as there are typically more damaging attacks, and hostile players won't be forced to attack you.
Against Hunters, Mocking Blow can be extremely useful to keep their pet attacking you during a freeze trap, breaking it, thus negating most hunters' main kiting technique. However Challenging Shout is perhaps a better option as it is can be used from any stance and is an AoE ability (which means you do not have to change your target) and costs half as much rage as Mocking Blow.