Superheroes were and still are an important part of me. Reading Spider-Man and Hulk are the reasons I wanted to make comics and Marvel is the direct inspiration for my Jonny and Valve series'. I never really got into manga as a kid mostly becasue they all looked the same to me. Where as every book Marvel put out at the time had different art styles (to me anyway). And probably more so today, since comic artist are taking a more cartoonish style (which I love).
I've obviously grown a lot since then and can appreicate the more down to Earth mangas, though I still lean towards the superhero genre. I just like seeing extrodinary heroes and villains throw different powers at each others. And there's people who can take simple ideas, like superheroes, and make interesting things from them.
Also I kinda like the ongoing nature of American comics. To me it's always something to forward to every week. I really think you should only end a series if the creators have to. I just hate when writers and editors take continuity too seriously. Like, why are you bringing up this one detail from 150 issues ago? Just get on with the story! JEEZ!
As for defining the superhero genre. Well, I guess the general consenus is American characters in spandex fighting in large cities. But a lot of animes like DBZ on Attack on Titan kinda look a little superhero-esque to me. So I don't think it's only an American thing, just that America has the "pure" superhero genre. I guess I'd define the superhero as more of a sub-genre of crime/cop stories. That's what superheroes are, people who decide to take the law into their own hands because the police just can't handle all the crime going on. Whether they have superpowers, gadgets, or just a mask is up to the writer.