Diminishing returns
Diminishing returns means that certain spells and abilities become less effective against a target if used frequently within a short period of time. While diminishing returns mostly applies to player characters, some abilities also have diminishing returns in PvE. The tables below contains a list of abilities that are affected by DR, and under what circumstances. In addition, DR categories are listed. Effects within the same category share diminishing returns.
When an ability with diminishing returns is used against a target in PvP, the first effect has full PvP duration. If the same category of effect (e.g. root) is used on that target within 18 seconds, that effect's duration is reduced by 50%. On the third use, the duration is reduced by 75%. After this, the target is immune from all effects in that category for the next 18 seconds. Note that when used against another player, crowd control effects will only last for their PvP duration, often far shorter than the duration stated in the ability's tooltip. The maximum PvP duration on Retail is 6 seconds unless buffed by Evoker's 
[Oppressing Roar].
Spells must be used on the same target within 18 seconds of the * end* of the duration in order to be diminished. In other words, if a target hasn't had a root spell active on them for more than 18 seconds after the root debuff expired, this category of diminishing returns will be reset on the target, and the next rooting spell will have full effect.
Diminishing returns for knockback effects and taunts are a special case with different timings. Knockbacks are immediately immune after first usage, and reset after 10 seconds versus 18 seconds. Taunts that force aggro from mobs or pets reset after 18 seconds like usual, but they only become immune after fifth usage instead of after the third.
In PvE
Many types of crowd control also suffer from diminishing returns versus NPCs, usually stuns and taunts. 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. A creature's own crowd control ability may also have diminishing returns on players, but not always.
In Classic
Diminishing returns in Classic WoW resets dynamically between 15-20 seconds, instead of a static 18 seconds.
Crowd controls in Classic Era may break early due to heartbeat resists.
Categories and abilities
The tables below has been sorted out into diminishing returns categories for their respective game expansions. Each ability shares diminishing returns with the other abilities it has been listed with, and no others unless otherwise stated.
Data for item effects such as 
[Frost Grenade] are currently not included. These can not be used in rated PvP, but still triggers a diminishing returns effect elsewhere.
The War Within data
Wrath of the Lich King Classic data
Some spell tooltips might refer to the Retail version.
Patch changes
Patch 9.0.1 (2020-10-13): Diminishing returns now affect secondary stats. (Crit, Haste, Mastery, and Versatility) 
Hotfix (2015-03-03): [...] So, we made a recent hotfix that changes the duration of diminishing returns to 18 seconds. [...] - Patch 6.0.2 (2014-10-14):
- Reduced the number of Diminishing Returns (DR) categories.
- All Roots now share the same DR category.
- Exception: Roots on 'Charge' types of abilities have no DR category, but have a very short duration instead.
- All Stuns now share the same DR category.
- All Incapacitate ("Mesmerize") effects now share the same DR category, and have been merged with the Horror DR category.
- A list of which Diminishing Returns category each crowd-control ability belongs to can be found in the forum thread titled: Diminishing Returns in Warlords of Draenor (archived version)
Patch 5.4.0 (2013-09-10):
- There are now diminishing returns on Vengeance gains while tanking multiple targets. Each additional target grants progressively less Vengeance.
- Area knockback effects now all share a diminishing return category. After a player has been moved by one of these spells, the player will be immune for 10 seconds. This new category applies to:
[Fellash], 
[Gorefiend's Grasp], 
[Glyph of Explosive Trap], 
[Thunderstorm], 
[Typhoon], 
[Ursol's Vortex], and 
[Whiplash]. 
[Divine Star]'s healing is no longer affecting by diminishing returns when used on multiple targets.
[Halo]'s healing is no longer affecting by diminishing returns when used on multiple targets.
- Hotfix (2013-10-11): We've seen some confusion recently about exactly which crowd control effects share diminishing returns. To hopefully clear that up, we're providing the following, categorized lists of all of the CC effects in the game. [...]
- Patch 5.3.0 (2013-05-21):
[Ring of Peace] had been redesigned. The ability forms an 8-yard sanctuary around a friendly target for 8 seconds, instantly disarming and silencing enemies within the area-of-effect for 3 seconds. Enemies that use abilities other than auto-attacks or cast spells on allies within the Ring of Peace will be disarmed and silenced for 3 seconds. Disarm and silence effects are subject to diminishing returns. - Patch 5.2.0 (2013-03-05):
[Warbringer] now reduces the target's movement by 50% for 15 seconds (8 seconds in PvP), in addition to its other effects. The 3-second stun/knockdown is now in the diminishing returns category for stuns (same as 
[Shockwave] and 
[Storm Bolt]), and not the proc stun diminishing returns category. - Patch 5.1.0 (2012-11-27): The
[Growl] pet ability will no longer cause diminishing returns against bosses that are immune to its effect. 
Patch 4.2.0 (2011-06-28):
[Deep Freeze] and 
[Ring of Frost] will cause diminishing returns on each other, in addition to the spells they normally share diminishing returns with.
[Sin and Punishment]'s 
[Vampiric Touch] backlash fear effect no longer has diminishing returns.
- Patch 4.1.0 (2011-04-26):
- The root effect from
[Shattered Barrier] now shares diminishing returns with the root from 
[Improved Cone of Cold]. 
[Charge] and 
[Intercept] no longer have diminishing returns on their stun effects.
- The root effect from
- Patch 4.0.6 (2011-02-08):
- Hunter Pet Fix: Ravage no longer counts toward stun diminishing returns.
- [Charge] now shares diminishing returns with stun effects.
- [Charge]'s stun continues to not trigger diminishing returns for Protection warriors who have the [Warbringer] talent.
- Patch 4.0.3a (2010-11-23):
[Improved Polymorph] now correctly shares diminishing returns with controlled stuns. 
Patch 3.3.0 (2009-12-08): Taunt Diminishing Returns: We've revised the system for diminishing returns on Taunt so that creatures do not become immune to Taunt until after 5 Taunts have landed. The duration of the Taunt effect will be reduced by 35% instead of 50% for each taunt landed. In addition, most creatures in the world will not be affected by Taunt diminishing returns at all. Creatures will only have Taunt diminishing returns if they have been specifically flagged for that behavior based on the design of a given encounter. - Hotfix (2009-09-09): We just deployed a hotfix to modify
[Holy Wrath] so this spell has the same diminishing return mechanics that other player stun spells and abilities currently have. This will prevent Undead creatures that we wish to be susceptible to stun from being permanently stunned if you include enough Paladins in your raid who then rotate their Holy Wrath casts. This will affect the difficulty of Anub'arak, but we didn't want the "right way" to defeat this fight to involve bringing five Paladins who used 
[Glyph of Holy Wrath] on their Holy Wrath spell. - Patch 3.0.8 (2009-01-20): All Silence spells now have diminishing returns. This includes:
[Arcane Torrent], 
[Garrote] silence effect, 
[Improved Counterspell] effect, 
[Improved Kick] effect, 
[Silence], 
[Gag Order], 
[Silencing Shot], 
[Spell Lock], and 
[Strangulate]. - Patch 3.0.2 (2008-10-14): Melee mitigation stats also have diminishing returns.
Patch 2.3.0 (2007-11-13):
[Blind] is now a physical (no longer a poison) attack. Reagent requirement removed. Now shares the same diminish category as 
[Cyclone] and is now diminished in PvE as well as PvP.
[Riposte] is now subject to diminishing returns in PvP.
[Frost Shock] is no longer subject to diminishing returns.
[Unstable Affliction]: The silence from this ability is now subject to diminishing returns.
[Disarm] is now subject to diminishing returns in PvP.
- Patch 2.1.0 (2007-05-22):
[Entrapment] (Survival): Effect duration reduced to 4 seconds and is now subject to diminishing returns in PvP.
[Frostbite] effect is now subject to diminishing returns in PvP.
[Death Coil] is now is now subject to diminishing returns in the Horror category.- Reputation: Players will experience diminished gains in reputation from killing outdoor mobs while in raids larger than five players.
Patch 1.9.0 (2006-01-03): The stun effect's duration no longer diminishes or is diminished by controlled stun abilities and spells:
- Patch 1.4.0 (2005-04-19): A handful of spells were added to the list of spells subject to diminishing returns in PvP. It is our goal to set as few spells as possible to be diminished. We've identified this short list of spells as having durations and cooldowns sufficient to almost permanently slow or immobilize targets but without adequate controls or other limiting factors. 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%. On the fourth attempt, the target becomes immune to the spell. 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. The list of affected spells is:
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