I think the main thing about Spider-Man (and most Marvel characters) is that people can relate to him. A lot of the Marvel characters have flaws, so while being super human, they are still human at the same time. If you get what I'm trying to say. People still know what these heroes are going through. Whether it's Spiderman going through changes as a teenager and having to make choices for himself or the X-Men being discriminated against because they were born different.
Not many people know what it's like to have your parents killed and still have a mountain of money to cry into. Even fewer people know what it's like to be an alien with almost god like powers. Maybe that's why they gave him a kid in that last movie of his.
The more the audience can relate with the character, the more popular they will be. Seeing Ironman build and test the suit himself will always be better then just watching Bruce Wayne go shopping in his company basement and taking what he needs.
Sorry, didn't mean to turn this into a Marvel vs DC thing. But I think that's why characters like Spiderman are popular.
__________________
-= Reality Bends To Desire =-
|