Thunder Clap is useful for tanking when a warrior needs to multi-tank a large number of mobs.
Be careful using this ability around crowd controlled mobs, as the damage it inflicts will likely break the crowd control effect. If possible it is best to apply Thunder Clap before crowd control, or to pull the mobs you wish to Thunder Clap away from the sheeped/sapped/trapped/etc. mobs. Also be careful of using this around a mob your group's hunter is trying to draw into a Freezing Trap, as it may make it more difficult for the hunter to establish aggro on the mob.
Thunder Clap can be useful against melee PvP opponents, as it reduces their ability to inflict damage. Its AoE effect against opponents makes it especially useful in large battles.