in 13 hours you should've hit the level cap anyway (it's 30).
don't bring Diablo up, because it works exactly the same. the cool thing about Diablo was its LAN, where you could start new games with the same character over and over again, jump to higher difficulty settings. but the core game stayed the same, you get to Hell, kill Diablo, game over.
anyway, 4 characters = 120 levels, the game's structure is non-linear, so you'll actually benefit from playing through it at least four times.
but if you don't care about the story, about your characters, about anything other than "DING", I guess this game isn't for you. that's too bad, but what do you want from the devs? to make a new game that suits your taste better?