For one thing -- well, you can define an epic as a "lengthy narrative poem." You can define a lyric as a "short poem expressing emotions." But you have to define a sonnet as "a poem of fourteen lines using any of a number of formal rhyme schemes,". Superheroes face much the same problem.
You can define science fiction as that branch of fiction that attempts suspension of disbelief by appeal to the authority of science. And fantasy as, hmm, that branch of fiction that attempts suspension of disbelief by appeal to the authority of folklore and legend. But superheroes, you have to start talking about superpowers, funny costumes, secret identities and crime-fighting. (Come to think of it, modern setting is also characteristic. I wonder if an ancient or medieval superhero is really feasible.) You can get recognizable superheroes with mitigated forms of those, or missing one or two, but they clearly will be less typical forms.
I was thinking that one aspect was a diversity of origins. You can have Iron Man, but if all your characters wear high-powered armor, you have mecha, not superheroes. You can have Doctor Strange, but if all your characters are wizards, even if they wear fancy get-ups, you have urban fantasy, not superheroes.
DC and Marvel of course accreted a mad medley of origins over decades. Many writers of their own little superuniverses tend to introduce a metaorigin -- but not an common origin. Once the metaorigin started to produce supers, they actually gained powers in many of various ways, and getting many and various powersets.
BUT -- that variety would preclude a superhero story where there's only one or two superpowered people. I have run across those. Hmmm. They do tend to be oddball origins that are clearly not typically fantasy, but if science fictional, very soft science. Perhaps I have not seen enough to judge.