One Punch has got something pretty good happening under the hood, and the scene of Mumen rider fighting the Sea King teared me up.
Nonetheless, the meat of what the series is doing is put out there right in the first episode, and it's really just hammering that point in. In order for a person to evolve, feel as if they own their life, they need conflict. As the story goes on, perhaps the most interesting it does is let on (in very subtle ways) just how much Saitama is being harmed by not getting the conflict he desires.
Beyond that, the manga has done what I thought was next to impossible and made comedy of errors funny to me.
You consistently have Saitama and the main-ish antagonist run into each other, and each time they don't know who the other is, and brush each other off (sometimes literally). And none of it seems forced or coerced because it's as if the entirety of the story has been written just to make these scenarios happen.
As if One started with these scenes, and wrote the rest of the story around them.
Detroit Metal City only wishes it was this natural.