I liked this a lot and really had fun watching it. I want to make this clear before I start ranting.

I think the writing is really weak for a drama of this caliber and with these mains. This story would never have worked if we did not have Jo Jeong Seok who could make you fall in love with him even if he plays a brick or a doorknob, and Go Gyeong Pyo who is so lovable and cute you can not help but feel sorry for him here, and of course Ms. Gong Hyo Jin who can make you root for any character she plays and maintains excellent synergy whomever she acts with.

Besides the mains, I think the only characters I liked were Hong announcer and Nurse O. They at least had some semblance of character to them (probably brought to life by their acting, not writing).
When you think about it none of the side characters had any character at all. they were just treated as a cog or a force to move the story in a certain way. they were never given any depth.

Think about Kim Tae Ra. Only reason we do not think how can a mother act this way is because we are used to these kinds of mothers. But unlike regular kdrama mothers she doesn't have any redeeming quality. Because Go Jeong Won is such a nice and kind character and because he lets her mother gets away with things we assume she must also has some good qualities but is there really? Why is he acting like he is her loving mother when nothing she does ever gives us any indication that she is anything but a self serving egoist.
At least with his father they did not tried to coat it with anything and used the father for what he is; a lever.

And not only the side characters. All of the mains have unexplained but assumed characteristics.

If you disregard and ignore Lee Hwa Sin is played by Jo Jeong Seok you'd have a near perfect example of a standard korean drama villain. Just imagine there is no scenes to show his side of the story (and even if you show some, maybe most of them too) he is nothing but a cold hearted and manipulative person with a narcissistic personality disorder.
Only reason we felt the start is shaky but then got into the story is because he is played by Jo Jeong Seok. If he was played by someone less lovable, less approachable then we would have dropped this series in a second. Well probably not me, I love Gong Hyo Jin too..

Why is Go Jeong Won such a good person? nothing in his life or background gives us any reason to expect or believe someone can grow up to be such a person coming from a family and background like his.

A lot of hooks were cast and left forgotten. What happened to mom-crush of Pyo Chi Yeol? He was so cute finding a mom figure in Bang Ji Yeong and then... puff; gone.

A lot of problems with pacing also. Realized who she will pick at 15-16 and waited till ep 20 something for her to acknowledge her decision.

Again; I like this drama. And quite possibly may rewatch it even tho I will get triggered and rage on stuff. Just like I do for most of Kim Eun Suk dramas (Tho so far Goblin is excellent *fingers crossed*).
For me the success of this drama is directly tied to the successful and pro cast and nothing to do with the script.