I recall reading somewhere that there are basically only 7 stories in all of fiction. Everything is just variations on these seven stories. So really, if you look at it that way, there probably hasn't been anything significant added to the world of literature since the ancient Greeks.
I heard William Friedkin say there are two stories only: the crucifixion and the odyssey. Maybe boil it down to one: Gilgamesh. I had the officiant read Gilgamesh at our wedding.
I am not a comic books historian, but I think Stan Lee's main contribution to the genre was in making characters relatable and vulnerable. Spiderman might have amazing abilities, but Peter Parker still has to worry about pimples and zits and his job at the Daily Bugle and looking after Aunt May and a lot of other things that normal people have to worry about too.
Poor Peter. Who is that again ? Is that his real name ?