For some reason, Superman seems to be held to higher standards on the subject of secret/super identities than other superheroes. No one ever says, 'Peter Parker was a nerdy kid. He can't possibly be Spider-Man, attract a good-looking gal, work in a newspaper, etc.' And no one gets hung up on whether his nerdiness is a disguise.
As far as superhero stories, what's appealing is of course that aspect of wish fulfillment. I mean, you start out reading them as a kid, and a couple things jump out at you - there are heroes out there, and you wish you could run into a phone booth and change your life, or be like Peter Parker and put on a mask and become a hero.