Superman's design was based on strong men from circuses that used to be prevalent during the Golden Age. But his design has many resemblances to certain depictions of Micheal Archangel. He is the humbler of all evil, he carries great power but always use in the name of a greater good. He fights the fights that we can't so that one day we can finally save ourselves.
Superman is willing to put many things in the line to protect humanity, even his own life. That's part of his charm and one of the reasons why the Superman=Jesus analogy just doesn't work. He isn't anywhere similar to Christ in theology.
The Sun (his source of power) is also a symbol of greater good and the Idea of Good. Which Plato relates to the inner greatness that all living beings should reach.
On another point, he is the son of a farmer and is very related to the natural process of life, which makes him part of Earth even if he himself comes from a very different place.
He is also a reporter trying to find truth anywhere he goes (even putting himslef at odds with the Planet when the paper became a source for tabloid celebrity gossip in the New52, which was a very interesting thing to do).
Finally, he can see your soul. This means that he values life even if he understands that death is imminent (which may explain why the guy is so against killing and yet doesn't preach about in Hitman). This is important, specially when you consider that he is also trying to represent justice.
Most of his problems comes from very human places. Such as self doubt, a love hate relationship with his kryptonian side and the struggle not to fail to his comrades or himself. And sure, a huge interplanetary fight that can be seen across galaxies are always an spectacle to watch.
Superman is one of the favorite character for fantasy writers for a good reason. He is fun to write, he isn't limited by many things other than the cosmos itself and it's own very humane nature.