Great handling of the plot, the way it unraveled is rarely seen in series, it had very smart decisions by the writer and no uneccessary flashbacks and excessive explaining that make the writers seem like they think the audience is dumb. Great cinematography. The acting was superb, even the many crying scenes which usually annoy me, here felt necessary. Even though it had the trope of the rich chaebol it was handled so perfectly that it didn't matter. Every episode kept me entertained. Every side character was shown in ways that let the viewer know their every thought and motive, I cared about them and understood their points of view. Great "villain" because his actions were realistic and not just for the sake of being evil, the scariest villain is the one that we can imagine being around us in real life.