Shadow Power

Shadow Power is one of the priest Shadow Specialization bonuses. It increases the damage of all the priest's spells by 15%, and the critical strike damage bonus of all the priest's Shadow spells by 100%. So for Shadow priests, offensive Shadow spells will crit for 100% extra damage, compared to the 50% extra damage that a critical hit will grant if the same spell is cast by a Holy or Discipline priest.

To sum up:
 * A Holy or Discipline priest casts a Shadow spell that deals 100 damage and crits for 150 damage.
 * The same spell cast by a Shadow Priest will deal 115 damage and crit for 230 damage.

So this passive bonus provides a considerable boost to damage output just for selecting the Shadow tree.