From Diablo Wiki
Overpowered (or OP) is a term commonly used to complain or accuse a skill, item, monster, or other game feature of being unbalanced and too strong. Whether a feature is OP or not is often a contentious issue and much digital ink is spilled by players arguing about the matter.
Features that are legitimately overpowered are usually nerfed in patches or hotfixes, though of course those patches themselves are then debated as appropriate or not.
 Overpowered in Diablo 3
A brief selection of game features that have been considered or at least argued as overpowered.
- Spin2Win Barbarian build is the fastest killing build in the game, largely thanks to the Sprint: Run Like the Wind rune effect.
- The Demon Hunter skill Vault: Trail of Cinders was bugged to deal 1500% weapon damage in v1.04. It was debugged/nerfed down to the intended 300% damage over 3 seconds in Patch 1.0.5.
- The Demon Hunter skill Elemental Arrow: Nether Tentacles was very strong at release, with each projectile able to land numerous hits. The skill was nerfed to reduce each orbs maximum hits to 2.
- The Wizard skill Energy Twister: Wicked Wind was nerfed by lowering the proc rate in Patch 1.0.4. This weakened but did not kill the build, since other rune effects in Energy Twister proved adequate, though lesser, substitutions.
- Black weapons are bugged to deal extra damage, but not so much more than it's generally considered overpowered.
There is a Barbarian skill called Overpower which is, ironically, not considered especially powerful.
- Yet more ironically, it once was very OP, and the rune effect Crushing Advance was nerfed in v1.04 to remove the potential to proc repeatedly from some types of attacks, which could make the Barbarian basically immortal while the skill was in effect.
- See more Reference terms.