Difficult to say what's wrong with this story, cause it's pretty much everything.
I would say the story has it's low points in the beginning but it's bearable. As soon as the narrative changes halfway through, it becames the most cliché and boring kdrama ever. The fights hold absolutely no meaning, our favorite secondary couple also starts to have the same fights over and over again, to the point you start to wish the guy would actually move on to a better person... it's a disaster. Kept watching cause i liked some of these actors, but I seriously can't handle how horribly written this was though.