3. He Has No Personality
This one is a no brainer. Superman is the personification of everything nice and that's as far as his character development goes. Like I've mentioned before, he's good because it's just who he is. He has no character arc where he learns a lesson or has to go through a great loss to realize his full potential. Superman is just good all the time no matter what is happening to him or what the circumstances may be. It's this part of his character that is most alien. Yes, he isn't a human and shouldn't act like a human but he needs to have some link to humanity that presents a challenge he must go through in order to develop. He may be fun to watch in his movies but there is no deeper significance that keeps them lingering in your memory. Superman is too distant and too perfect to be a compelling hero.