I loved this show and thought it was about what it means to be human, and what that entails -- love, loyalty, hardship, betrayal, and forgiveness. Above all, it was about how to remain a decent human being regardless of what befalls you, as Dong Hoon had done. I was happy interpreting Dong Hoon's relationship with Ji An as platonic because I am not sure he would have been quite as decent if he were fooling around with a girl young enough to be his daughter.
You said there was controversy over casting IU as the female lead. Was it because she was, as I'd said, young enough to be his daughter and fans assumed she and he would be playing lovers? If so, then the fans were being silly to make assumptions without seeing the show first.
At any rate, I was glad this relationship remained platonic, and that Dong Hoon got back together with his wife. What troubled me was that at the very end they tell us his wife is living in America with their son for a year. Meaning, he's not really with her. Now, this couple has a lot of work to do to mend their relationship, yet she flies off to America for a year? Most fans were frustrated because they thought Dong Hoon's relationship with Ji An was inconclusive; but I thought it was his relationship with his wife that was inconclusive. After all, we can't be sure they're truly happy together if she's living halfway across the planet from him. Moving her to America was just such an odd and unnecessary plot point to toss in at the very end.