You can see how he changed on the surface. But at the core of it all, I think Superman has remained the same - a character with incredible powers but almost superhuman humility and restraint.
The way you challenge Superman is by having things happen very, very quickly in different places and then asking, 'Who does he save first? What powers must he use to save each person or stop each disaster?' That's one of the ways you make him interesting beyond the thematic and moral issues that make Superman.