First of all, I think one of the most important thing for a drama is the pacing. How many times have you watched a drama and it's only when you finish the 3rd or 4th episode that it's starting to get good ? In Goblin in episode 1 you are introduced to all main characters without an "in your face" introduction. The world building is enough for understanding the story, but keeps just enough mystery and details to catch the attention. The music is amazing and, from I remember, well timed between scenes. In some dramas you feel like by episode 2 you heard them all already, in this one it takes more and the ambience is fitting (plus I think they are very good, even though it's not really my type). The casting is phenomenal and every character have a story that is relevent and interesting. The second roles are not just a distraction from the main cast's story. I could talk about Goblin all day, but finally, The plot and ending, you dont see it comming and it's more and more rare these days I think, that's all I'm gonna say.
I'm a grown man in my twenties and I cried many times and that's what I want from dramas, to reach my emotions. whether it was from sadness, hapinnes or both is for you to find out.