The original Superman may have been popular for all his neat super powers, but I think the character has endured because of the rich and dark past that led to his current superhero status. Sure, he's a boyscout with near invulnerability, but his parents, and nearly every other kryptonian, were violently killed when their homeworld exploded. So now there is a heavy feeling of loneliness while he continues to battle with Krypton's ghosts (like kryptonite or general zod). For this reason, I find his character to be that much more interesting and heroic. He has been placed in an impossible situation on an alien world. There are so many reasons why someone in that situation might give up and spiral into depression, but instead he chooses to be a hero. He chooses to lead a life in pursuit of life and happiness. He defends those who cannot defend themselves. There's also a Jesus comparison in there, but for me it just boils down to a hero that one can look up to, not just because he has neat powers, but because he represents personal strength and a sense of hope in otherwise hopeless situations.