Agreed. As a superhero, he's pretty dull.
I think I know why Superman is so dull: no inner conflict.
Superman does good out of a sense of public service - and that's it. Batman, on the other hand, is conflicted between his sense of duty to Gotham and his desire to avenge his parents.
Batman, too, frequently faces instances of gray-and-gray morality. Superman doesn't: he's the do-gooder, and his job is to vanquish the evil-doers. End of story.
Gray-and-gray morality is also why a film like
Silence of the Lambs - the original - is so good. Can Clarice work with one sociopath to stop another? And she can only stop him at the cost of letting Hannibal Lecter escape. There are no "good endings".
Thoughts?