I used to read the manga and last weekend I finally watched the anime, binged on it
My only criticism would be that each episode essentially plays out the same, with the villain being continuously built up throughout the episode, unable to be beaten by the other heroes until Saitama comes along and beats them. Although, I didn't have much of a problem with this, however repetitious it may be. While Saitama is practically unbeatable, the other characters in the series are not, and so you do feel a bit anxious and worried for them. I really like the action in this anime, and the way they make fun of shounen cliches.
I do cringe every time Puri Puri Prisoner is on screen because I feel he's kind of offensive, but he's also a hero at the same time, and S Class. I'm not really sure if his character is homophobic or not. He's also a little rapey. He isn't that important to the story, though, and he's used mainly as a joke, so whatever.
One Punch Man is a well-needed and refreshing take on the whole superhero genre. Instead of our hero being cheery, pure of heart, and with his own personal tragic backstory, we get just a regular joe who happened to get super powers. He's casual and laid back like other Shonen heroes (like Goku), but for a different reason. It's because he's so confident in his own abilities and strength that he can't help but not take anything seriously. (That, and he's a total airhead.) This carefree nature also brings out most of the comedy in the show, which I am 100% okay with. Most anime shows are sometimes somewhat hard to relate to with their comedy stemming from their culture. It's like watching Hot Fuzz when you're an average american. It's not as funny because it's not american style humor. One Punch Man breaks this cultural wall fairly easily, some examples including the slow motion dick punch with Sonic Speed or the borderline drag queen nudist, Puri Puri Prisoner.
Now, onto what a lot of the serious anime community loved about this show (other than the plot itself), the animation.
Animation in One Punch Man is beautifully done, from scenes where he tears out the Crab Mans Eye to when he fights the legion of the underworld guys. Animation of this level can easily be compared to Studio Trigger level of animation, the same studio who worked on Kill la Kill and Gurren Lagann. Almost every action scene is filled with vivid life and emotion, and on occasion the casual scenes get a little touch of quality as well.
My favorite character, other than main protagonist Saitama himself, would have to be Mosquito Girl Mumen Rider. He's a very relatable character in my opinion, his strive to be a superhero comparable to some of us when we were kids. I know I sure as hell wanted to be a superhero when I was a strapping young lad, and Mumen Rider personifies that feeling to the extreme.