5. He Doesn't Represent Anything
All of the best superheroes have something behind their actions that they stand for. Captain America fights for the love of country and the innate goodness that people can display, Iron Man fights to stop his own inventions from being used in a way he never desired and Spider-Man begins a quest for vengeance that turns into a fight to protect his loved ones. Superman protects people because he is good and being good is the right thing to do. That's the most basic character trait in the book and is literally the essence of being a hero. If you're not fighting for the greater good you're a villain or an anti-hero. In this way Superman doesn't really stand for anything. He just kind of exists with no greater backing for his actions and this lends his stories a tone of boredom. Heroes are only interesting because they have motives that intricately connect with them as characters that influence why they do good deeds. If you skip the "why" of a story all you're left with is the "what" and that makes any story tonally inconsistent and boring.