Percent

Often modifiers (haste, damage and so on) will be referred to by percent, with percentages over and under 100% (where 100% is accepted as the baseline comparison). For instance:

Adrenaline rush increases your energy regeneration by 200%

This means that your rate of energy regeneration is doubled (rate * 2.0). As it stands, Rogues gain: 20 Energy / 2 Seconds so Adrenaline rush would cause you to regain: 2.0 * 20 Energy / 2 Seconds = 40 Energy / 2 Seconds

The wording on abilities varies as well. Sometimes a modifier will be referred to as adds 50% damage, which means the damage is 1 + 0.5 = 1.5 times that of the original, in other cases, the wording is like in the example above; increases your energy regeneration by 200%, meaning the energy is 2.0 times that of the original rate.

In general, the following terms are equivalent: Your mount goes 2 times as fast Your critical strikes will do 1.25 times as much damage Your ability now lasts 1.15 times as long
 * Mount speed is increased 100%
 * Adds 25% critical strike bonus damage
 * Increases the duration of your ability by 15%

As of Patch 1.12, all modifiers have been changed to multiplicative (as opposed to additive). This means that an effect of +30% and an effect of +50% would not equal an effect of +80%, but instead an effect of +95%.