I might do this for other characters. I'm starting with Superman but I might make this a series of takes. Here we go:
10. He was originally written as a villain
In a story called The Reign of the Superman about a bald telepathic villain who gets his powers because a mad scientist experimented on him.
This idea got rejected and we got the Superman that we know.
9. He was the first DC character to break the fourth wall
There's an early 40's Superman story in which Clark has to get Lois out of the movie theater playing the Superman cartoon (which really was a show at the time) before his secret identity is revealed on-screen. Clark winks at the reader at the end of that one. 
8. Superman couldn't always fly
When Superman first debuted during the golden age, his only powers were “Being faster than a speeding bullet, more powerful than a locomotive, able to leap tall buildings in a single bound”. And the last one was only because earth had lesser gravity than Krypton, just like astronauts can jump higher on Moon. 
7. He was once the most powerful character in his universe
I already said earlier that golden age Superman was nowhere as powerful as he is now, but then we have his silver age version. His feats include moving planets with his breath, accidentally destroying a solar system by sneezing, and flying at 40 trillion times the speed of light (yes, much faster than flash).
DC rebooted their universe after the Crisis on Infinite Earths storyline and changed the power levels of many of their characters. It was only after this that characters like Kismet and the Presence were introduced.
6. There's a pink kryptonite which makes him gay
5. He's won every canon fight against Batman
Despite what some Batman fans claim, Batman has never won a canon fight against Superman. 
Batman has won in comics like The Dark Knight Returns and maybe even Injustice, but they were all set in alternate universes. Batman's only canon victory might be in Endgame, but even that is debatable.
4. Superman is worthy of Thor's hammer
Marvel and DC had a crossover where he managed to lift it. 
He failed to do it a second time though. 
3. He learns stuff faster than (almost) anyone on Earth
People often underestimate his intelligence, but his brain can process information many times faster than a human brain.
Although Flash can think a lot faster than him, his memory is not as good as Superman's.
2. He's one of the best scientists on Earth
Going by feats, I personally feel he's second only to Lex Luthor. The tech that he's created is more advanced than anything that characters like Batman, Flash, and even Cyborg have made.
And let's not forget that the Hellbat armor that Batman used to fight Darkseid was forged by Superman in the sun. Or that he's managed to make an AI of himself.
1. He's the most iconic comic book superhero of all time
He was one of the first superheroes and he's indirectly the inspiration for the majority of the ones you see today. He's also the first character that a lot of people think of when they hear the word superhero.
I might end up doing this about another character if this gets enough views :)
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