What makes a K-drama worth watching?

What is it that you like in a K-drama?

What is your favorite K-drama and why?

A kdrama worth watching is a kdrama that pulls me in from the beginning to the end.

I like well fleshed out characters who have meaning and reason in what they do, they don't exists just for one reason. 

One of my favorite kdramas is That Winter, The Wind Blows. I like it for the chemistry, characters and development.