He is a man who gained the power to one hit kill any enemy he punches because of his exercise regime: 100 pushups, 100 sit ups, 100 squats and 10km running every day!
He became strong, and he lost his hair.
Also he is rather disappointed at how everything he punches dies in one hit. Episode 1 has him dreaming of a fight where his enemies actually pose a threat to him.
It takes most anime protagonists an entire arc to beat their foe. It takes Saitama one punch, much to his own chagrin.