Board Thread:Questions and Answers/@comment-25041815-20140613114853/@comment-4651179-20140613231351

Superman and Hulk never lifted Mjolnir because they were worthy.

Superman wielded it because Odin temporarily broke the worthiness enchantment. And Hulk has "lifted" it by grabbing Thor's hand while Thor wielded it, and one time the worthiness enchantment was temporarily broken.