I find many of the questions in this thread fascinating. Julian's initial question about the centrality of WWII to many superhero universes seemed easily explainable, given that the "Golden Age" was such a source of superheroes and that most writers and artists working in the genre since haven't exactly been pioneers. For the last few decades most of the creative people in superhero comics grew up as hard-core fans and suffer from something akin to historical/cultural myopia on the subject.
It seems right now that there is a greater variety of superhero and related comics being done than ever before and I would be willing to bet that very few of them are hung up on WWII as a starting point. Of course, I buy hardly any of them, so I could be wrong.
Broader questions about what constitutes a superhero are always interesting to me. On the subject of fantasy heroes (and now that I know Pacheco --pretty is right!-- was involved in Arrowsmith, I might actually check it out) I also think that sometimes, unfortunately, the medium is the message. When Thomas, Buscema, Windsor-Smith, et al brought Conan to Marvel, they seem in retrospect to have turned Conan into a superhero of sorts (I mean, if the Hulk and Submariner are superheroes...). That is, as a Marvel comic, Conan had to be a superhero. Ditto Sgt Fury. Ditto Luke Cage. Ditto Chang-Chi. Ditto attempts to do Doc Savage, the Shadow, etc.
Another aspect of most superhero comics that taints all efforts is the persistence of the corporate serial model of production. Characters are envisioned as infinitely renewable properties whose adventures are published or released on a regular basis. Art teams, writers and even publishers can be changed without any major damage done to the property. I know this isn't the case with many creator-owned projects and mini-series/graphic novels, but there is enough of it still around to have an effect. So even the most arcane Vertigo or independant series seem like superhero epics to me sometimes. Ditto many popular manga and manwha.
I'm on the record on
another thread about "different" superhero comics I still enjoy as an adult (outside of things like Silver Age Superman and the comics I grew up with in the 70s), but I think that claims for superhero comics as a universally enjoyable genre with infinite possibilities are over-stated. While not exactly in the Destroy All Superheroes camp, I would say that there is only so much I can empathize with a character if he (to paraphrase Howard Chaykin and others) is wearing his underwear outside of his pants and can make icecream.
And as for thriving (meaning to grow), there are definitely non-DC and Marvel companies that fit that category. Most of them publish Manga.