User blog comment:Kashicorn/What champions do you think are OP?/@comment-81.83.34.61-20121102045940/@comment-4022742-20121102131939

Its op when it is vastly superior over similar skills by other champions. This is what makes Amumu's Ultimate OP, compared to the similar ulti's of Galio and Nunu.
 * Nunu's still allows movement, just really, REALLY, slow movement. But as a trade off you can't really take any other action during casting and your ult can be broken from the outside.
 * Galio's pulls everyone towards him efectively keeping them out of combat for the duration. But as a trade off you can't really take any other action during casting, you take the damage yourself and it can be broken from the outside.
 * Amumu's stuns everyone, keeping them out of combat for the duration. However he himself is still free to do his damage and set up a perfect Q in the time his ult lasts and can't be broken from internal or external sources... This makes him OP in comparison