Re: The "realism" thread
I've always thought that had to do with the characterization, as well as the glasses. Clark Kent is mild-mannered, a bit of a coward; Superman is a powerful, superhero.
Also people can look different when they wear certain clothes. When Superman shows up, he wears a blue outfit, with a red "S" and a red cape. When Clark shows up, he wears a suit, tie, and glasses and blends right in.
Maybe that's the point. No one looks at Clark as being Superman because his features are so ordinary, they see the clothing and say "That's Superman." or "That's Clark." depending on what he's wearing.