Unlike most generic shounen action heroes, One-Punch Man doesn't start off weak, he starts off bloody strong, and remains bloody strong.
Here is a quick summary taken from MangaFox:
"Onepunch-Man follows the life of an average hero who manages to win all of his battles with only one punch! This ends up being the cause of a lot of frustration as he no longer feels the thrill and adrenaline of fighting a tough battle.
Maybe all of his rigorous training to become strong wasn't worth it. After all what's so good about having an overwhelming power?"
It has a mix of action, humor, and down-right bloody awesome action scenes; some actions scenes are so bloody cool that you can make them into an anime gif.
Ex:
Dat color.
Dat epicness.
