It's not just that the source of superpowers will determine a lot of things about the world. It's going to influence the world subsequently.
Alien artifacts that landed on earth distribute powers -- randomly or by personality. Mutant genes. Strange beings handing out marvelous abilities to carefully selected people.
Sometimes you will be able to morally evaluate a person by the powers he uses. (And what happens if he reforms, or is corrupted? Hmm?) Sometimes the correlation will be none, so that the person whose touch is poison is angelic, and the great healer a double-dyed villain.
Not to mention it will affect attitudes toward them. Imagine a world in which everyone dreams a dream of superpowers (perhaps based on their character) and then at the end gets to choose -- still caught in the dream -- and a tiny handful say Yes. I suspect that would be a world where superheroes are resented beyond belief.