"Superman was the start of the whole superhero thing. He had the superpowers and wore that costume with the bright colors and silly cape. It’s the costume that was different. Zorro didn’t have superpowers, Doc Savage didn’t have superpowers; they could just do things a little better than the rest of us. The Shadow could be a superhero because he could make himself unseen, and if he appeared in a comic book today, he might be a superhero, though he doesn’t really wear a costume. I’m not an expert on the Shadow, but I think he just had a dark business suit and a sort of raincoat and a slouch hat. Superman’s costume was different because of the bright colors, that silly cape, those red boots, his belt, and his chest symbol. I mean, it’s ridiculous, because you really don’t need a costume to fly or fight bad guys. If I had superpowers, I wouldn’t wear a costume. [...]
Although a costume isn’t required of superheroes, the fans love costumes. The characters are more popular if they wear costumes. (Don’t ask me why.) In the first issue of the Fantastic Four, I didn’t have them wear costumes. I received a ton of mail from fans saying that they loved the book, but they wouldn’t buy another issue unless we gave the characters costumes. I didn’t need a house to fall on me to realize that — for whatever reason — fans love costumed heroes."
-- Stan Lee, from his essay for What is a Superhero?, from Oxford University Press
(For the story on Lee's 1983 "centerfold" photo, visit Sean Howe's Marvel: The Untold Story blog.)