I win button refers to any skill that other players feel is overpowered in PvP combat. The suggestion is made that players need only one button on their UI, and can simply mash it to achieve victory.

Blizzard lampooned this attitude with the [Broken I.W.I.N. Button], which sometimes drops from player corpses in Alterac Valley.

Examples of "I Win Buttons"

Most so-called "I win" buttons are actually situational in use and/or have substantial cooldowns or other restrictions. Some have been hit with the nerf stick quite heavily since their introduction.

Shaman - Frost Shock
In early patches, Frost Shock was not affected by diminishing returns. This allowed any shaman to indefinitely kite any melee class to death. It has since gained diminishing returns on the snare.
As of Patch 2.3.0, Frost Shock is no longer affected by diminishing returns.
Paladin - Divine Shield
Combined with Holy Light and Flash of Light, Divine Shield and Blessing of Protection were often used together for a long period of interrupt free healing, especially with the Paladin at very low hit points. With the addition of Forbearance and Mass Dispel it has become much more difficult to string together these heals, though many players are still bothered by these spells.
Paladin - Hammer of Justice
Because of its ability to stun players for 6 seconds or the full duration of a ret paladin's rotation , some consider it an instant win if used on a non-healing player whoes trinket is on cooldown. Also of note is the longer range on this ability (10 yards) compared to the range of melee combat (within 5 yards).
Warlock - Death Coil
Still considered an 'I Win button' by some, Death Coil heals the caster, damages the target, and causes the target to flee in horror for 3 seconds. Most anti-fear abilities do not cancel the horror, and the horror is on a different diminishing returns timer than fear.
Mage - Frost Nova / Ice Lance
In early TBC, targets rooted by Frost Nova could often be hit for up to 4 high-damage Ice Lances, potentially critting for 2k damage each. Frost Nova has since been changed to have a high probability of breaking on damage against high-level targets.
Mage - Presence of Mind + Arcane Power + Pyroblast + some type of spellpower trinket
The famous "3 minute mage" scenario - when all the cooldowns are available, a properly specced mage can instantly cast a very high-damage spell.
Rogue - Cloak of Shadows
Many Warlocks, Mages, and Shadow Priests continue to consider this ability overpowered, despite its short duration. Used smartly, this ability is very powerful: however it is easily squandered by the inept.
Hunter - Bestial Wrath + The Beast Within
Beast Mastery specced hunters can gain complete immunity to all forms of CC, roots, fears, and snares, plus increased damage for 18 seconds with this combination. It can only be ended early with the death of either the hunter or his pet. Considering that many strategies to beat hunters involve snaring him to get in melee range, this can be very powerful.
Druid - Cyclone
Because it is undispellable, cyclone is the only form of CC without any counter at all (other than the PvP trinket). It has no cooldown, but has a cast time (and can thus be interrupted) (Note: As of v3.2.2 Predatory Strikes allows a druid to instant-cast a Nature spell after landing a finishing move in cat form), lasts only 6 seconds, and is affected by diminishing returns.