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In my opinion at least a champ should be nerfed if they fill at least two of the following criteria.
1) They are so good as to render champs that fill a similar role totally pointless.
2) They single-handedly push the metagame in an undesirable direction.
3) They have extreme damage potential coupled with extreme survivability.
4) They have a win-rate of 55% or higher.
5) They fill multiple team roles significantly better than most specialist champs do.
If two or more of those criteria aren't filled then the champ probably doesn't need to be nerfed. To take an example from DotA...
Vengeful Spirit consistently maintains a win-rate of about 60% which is quite high in any competitive game that's seeking character balance. So that's one of the four criteria. She pushes the meta-game in a pro-ganking direction which is desirable, so that criteria fails. While she has high damage output and likewise buffs her team's DPS substantially she's fairly smushy with little in the way of viable escape mechanisms. Finally, other gankers such as Shaker, Chaos Knight, and Sand King are all still quite useful. While Venge can play the role of an off-DPS for her team there are plenty of other heroes that do a much better job of it.
Only one out of five. Which is why the arguably best hero in DotA can get away with only extremely minor nerfs.