Taking the Shonen/Superhero Genre and flipping its tropes on their head. Showing them for how unmotivating, meaningless, and ultimately nihlistic their message ends up being.
While most shonen/superhero stories or power-fantasies, of the like - try to awe the viewer by giving them a sense of hope, One Punch Man shows you how that thirst for Power has absolutely NOTHING to do with anything that could be considered positive or optimistic. Instead it makes things downright depressing. The Lack of Competition or Threat makes obstacles seem like a chore and therefore makes Life uninteresting and deprives people from taking meaningful challenges. The truth about Power-Fantasies is that it makes people too comfortable in their own little bubbles. Vapid Escapism doesn't really offer motivation for solving real life issues, they become a distraction.
Since, Saitama can defeat any enemy that comes in his way with minimal effort, there is nothing left to do for him but engage in vapid escapism, indulge in consumerism and live a meaningless existence. Without challenge there is no growth and without growth, there can only be stagnation. And Saitama's biggest challenge becomes instead to break that stagnating cycle, but alas, he is unable to lose.
In terms of the characters, aside from Saitama, every side-character has some typical generic backstory that is put in contrast with Saitama having no backstory - getting his powers from doing 100 sit-ups, push-ups and a kilometer run. Genos being the biggest example of a Character that has tacked on backstory with complicated motivations and a lot to learn and grow, which puts him directly in contrast with Saitama for the vast majority of the show.
The ONLY other Character that doesn't have a tacked-on generic backstory that sticks out despite having little screen time is Mumen Rider. A comparatively weak hero with a heart of gold and the dedication of a Saint. Ironically the truly inspirational character in the entire show is the weakest character who barely accomplishes anything. This is runs perfectly with the themes of the Show. As power doesn't bring meaning, rather dedication does.
One Punch Man is a truly unique series from the 2010s Era. While most shows from this decade can't help but fall into tropes, cliches, or become parodies of themselves, One Punch Man succeeds by reversing the meaning of struggle in key structural ways that makes it an Intelligent Subversive Comedy.
