This drama could've been so much better,
As others reviewers have said, I blame most of it's failure in the screenwriter and the director. There were times that made you think they didn't even know what they were doing with their characters, and it translates in the acting. For example, Moon Ga Young was really good, but they made her character act the same way again and again and again and it became tiresome to watch. The same can be said for Woo Do and Kim Min Jae characters. Regarding Joy's, well, is a mix of her not having the acting skills and the poor writing of her character. Also, I didn't feel Woo Do and Joy had much chemestry, Woo Do and Moon Ga Young did.
The drama dragged, and even the parts that are supposed to move you, when they reveal things, never have the emotional pay off you want them to. Joy's character inmediately forgives him everything, so there's no big surprise that she also forgives his last betrayal. Also, the 5 year time jump with the time heals everything is crap and lazy writing. If you've hurt someone and they can't trust you anymore, you don't earn that trust back by staying again. You do it by being there for them without any pretentions or expectations, just because you care.
I wouldn't rewatch this at all, and I only started watching because I liked Kim Min Jae in Goblin, and kept watching because I was curious of Woo Do and Moon Ga Young's acting.