It's really not your typical ol' Shounen where the mc gets a new threat, needs to get stronger, eventually does, defeats the enemy, repeat. I think that Saitama is a sort of a satire to these kind of mc's. He doesn't really evolve and his personality doesn't really meet the criteria of a hero with high ambitions to the future - Saitama is just a hero as a hobby.
I think that One-Punch Man is like a breeze of fresh air in the whole Shounen scene. Like, Shounen is still great but having this comedic relief that kinda jokes on the Shounen mold is really nice imo.
Also, the fact that Saitama is op and doesn't have much development doesn't really make the story dull. There are plenty of side characters in the story that do try their best and do evolve in time and I like that the focus is not on Saitama 24/7. You as a reader, have the time to be introduced to new characters, even in battles and observe their hardships and motives as their own characters rather than just supporting characters. Then Saitama comes and you know it'll all be ok.
I recommend reading the manga a lot.
It's a nice, refreshing experience.