I think the definition of a superhero can be broader than just a bullet point checklist of characteristics like secret identity, kid sidekick, wonder dog helper, headquarters in the Amazon Rain Forest, evil scientist archnemesis, and so on.
I would certainly hope it would be broader than that list, yes, but then, I haven't seen anyone suggest such a thing.
As an aside, I always found it interesting that whenever certain writers like Ellis or Scott McCloud who write a great deal about the superhero genre's hold on comics, when listing other kinds of comics they'd rather see, they always mention the repeated litany of "science fiction, romance, westerns..." Like clockwork, they always bring up Westerns. I always found this hilarious, because it shows how embarassingly out of touch this perspective is with not just the reality of the comics industry, but from the reality of pop culture as well: name me one Western flick in the past TEN YEARS that's made anything resembling money, Kevin Costner vanity pieces aside. It's a once-universal genre cast into irrelevancy by the hold of disaster films and science fiction on big-budget movies, and by the hold of crime, tech and spy thrillers on the publishing world.)
They're simply listing variety, not intending to make claims that all the genres they mention are the most popular, or even that comics should only aspire to genres that make money as movies. I know Scott would like to see plenty of stuff in comics that wouldn't make money in Hollywood. So it seems odd to laugh that they'd mention a genre that hasn't been topping the charts in Hollywood (and Costner's last Western was over ten years ago, as it happens) when they weren't making any claim as to popularity, just to variety.
That said, westerns are still a profitable prose genre, though they sell far better outside urban areas than in them. There aren't really any "star" Western writers any more, beyond McMurtry, but the genre mill keeps rolling. And the genre's not fully gone from movies, either.
It is true that some characteristics define superheroes, just like some plot elements define science fiction: space travel, alien life, invaders from Mars, and so forth. However, if a story does not feature invaders from Mars or aliens, but only space travel, that does not make it any less within the confines of science fiction.
You seem to be arguing against something nobody's said. I've repeatedly said that not all the hallmarks of the superhero need to be present, merely that enough of them do -- though "enough" is a subjective point.
But LORD OF THE RINGS, for instance, has secret identities, super powers, an idealistic mission of good against evil and more, and yet would not be considered a superhero story by any useful definition.
The presumption that opponents for superheroes have to be in some way atypical comes from the fact that as the Marvel and DC universes are supposed to be exactly like ours except with fantasy elements, the appearance of said fantasy elements makes these characters unusual in our otherwise "real" world.
The presumption is your own, not mine. Bank robbers and muggers are not atypical, but are fair opponents for superheroes.
Sentinel robots are exotic to we readers, but they're part of the day to day reality for mutants in the Marvel Universe.
Not really, no. They're unusual events that date back ten years or less and have appeared only intermittently in that time; hardly a parallel to the magical elements of Arrowsmith's world.
But I'm not sure how far it's worth taking this -- you keep pointing out that there are exceptions to any rule, which is something I pointed out up front, so it's not as if we're disagreeing on that score. But I'd also argue that just because any individual superhero trope is not required, that doesn't mean that the absence of those tropes is irrelevant, merely not all-important.
And I'd continue to argue that ARROWSMITH isn't a superhero story, because virtually all of its tropes come from fantasy. And while fantasy tropes can easily be present in a superhero story, it still needs to have something that marks it out as a superhero story.
kdb