Diminishing returns

Diminishing returns means that certain spells and abilities are less effective against player characters if they are used frequently within a short period of time. The table below contains a list of abilities that are affected by DR, and under what circumstances. In addition, DR categories are listed. Effects within same category diminish each other.

When a spell with diminishing returns is used against a target in PvP, the first effect has full duration. On the second use of the same category of spell (e.g. slowing), that spell's duration is reduced by 50%. On the third use, the duration is reduced by 75%. Spells must be used on the same target within 15 seconds of the *end* of the duration in order to be diminished. In other words, if a target hasn't had a slowing spell active on them for more than 15 seconds, the next slowing spell will have full effect.''

Some abilities share diminishing returns&mdash;that is, it doesn't matter which one you use; the next time you use an ability in the same category on the same target, its duration will be diminished just as if you had used the exact same ability. These abilities fall into the same category.

Shared Diminishing Return Categories
Banish, Charge, Cyclone, Entrapment, Scatter Shot, and Mind Control are on diminishing returns with themselves.
 * Controlled Root - Freeze (Water Elemental), Pin (Crab), Frost Nova, Entangling Roots, Earthgrab (Storm, Earth and Fire)
 * Controlled Stun - Bash, Concussion Blow, Deep Freeze, Demon Charge, Gnaw, Holy Wrath, Inferno Effect (Doomguard summon), Hammer of Justice, Intercept (Felguard), Intercept (Warrior), Intimidation, Kidney Shot, Maim, Ravage, Shadowfury, Shockwave, Sonic Blast, War Stomp
 * Disarm - Chimera Shot - Scorpid, Disarm, Dismantle, Psychic Horror (Disarm effect), Snatch
 * Disorient - Dragon's Breath, Freezing Arrow, Freezing Trap, Gouge, Hex, Hibernate, Hungering Cold, Polymorph, Repentance, Sap, Shackle, Wyvern Sting
 * Fear - Blind, Fear (Warlock), Howl of Terror, Intimidating Shout, Psychic Scream, Scare Beast, Seduction (Pet), Turn Evil
 * Horror - Death Coil, Psychic Horror
 * Opener Stun - Cheap Shot, Pounce
 * Random Root - Frostbite, Improved Hamstring, Shattered Barrier
 * Random Stun - Impact, Revenge Stun, Seal of Justice, Stoneclaw Stun
 * Silence - Arcane Torrent, Gag Order (Warrior talent), Garrote, Improved Counterspell, Nether Shock, Silencing Shot, Improved Kick, Shield of the Templar, Silence (Priest), Spell Lock, Strangulate
 * Taunt - Angered Earth, Challenging Howl, Challenging Roar, Challenging Shout, Dark Command, Death Grip, Distracting Shot, Growl (Druid), Hand of Reckoning, Mocking Blow, Righteous Defense, Taunt (Pet),Taunt (Warrior)

Diminishing Returns in PvE
Many types of crowd control also suffer from diminishing returns versus NPCs. Diminishing returns do not count per player but per creature; every creature has its own diminishing return for each group of CC and all players in your party / raid will be affected by that diminishing return.