Please note that you can probably break any of the following traits of superhero stories to create one. But break them all? I think not.
A superhero story:
1. Is about heroes and villains whose distinctive clothing sets them apart from ordinary people.
2. Is about characters who are best known to the public by their pseudonyms.
3. Is set on Earth, usually in the present.
Now, what's interesting to me is that being super isn't necessary—costumed vigilantes like the Phantom and the Batman broadened the definition of "super power" to "exceptional human power".
Guardians of the Galaxy is about a group of uncommonly capable people who have to defeat a great menace, but if that's your definition, Firefly, Star Wars, Buffy, and Galaxy Quest are superhero stories.
Then again, I heard that to kids today, any book is a novel, so, fine, Guardians of the Galaxy is a superhero movie.
ETA: Something I said on Facebook: Green Lantern in space is a scifi cop story. Green Lantern on earth is a superhero story. Part of it is the surroundings: superheroes exist in a mundane reality, not a fantastical one. They are the fantasy element.