It was a superhero story the whole way through. It's just that (Sixth-Sense-like) they don't quite resolve it to that until the end. (Actually, the ending was the least superhero-y thing in the movie.) In fact, it was a comic book the whole way through. The typical superhero comic tropes were there throughout. The cinematography came as close to emulating comic book panels as is possible. The use of colors and shapes were identical to many classical superhero comics. They even point a lot of it out in the dialogue. Jack Kirby would have been proud.

And why did the fact that it was a superhero story piss you off? Did it break with the story in some way?