Well, this drama is undoubtedly a deserving hit in the area of kdramas. It's just that Kim Eun Sook is really one of the best screenwriters of kdramas, as she manages to create absolutely fulfilled and fresh stories, where even the secondary story-lines have their power. The only disadvantage of her works is that the last episodes become dragged and a bit too sweet but as it's something common in the dramas (the exception would be probably the best kdramas screenwriter Noh Hee Kyung, the author of That Winter the Wind Blows and It's OK, that's Love), I guess we should simply get used to it...
However, Goblin is my favorite Kim Eun Sook's drama and there are a lot of reasons. Firstly, here we find a really strong mystical atmosphere and an interesting world with its own rules (I cannot appreciate the perfect musical choice witch makes you really submerge into the fictional world). Also the humor side here is one of the best in the Korean area. Another wonderful thing is, obviously, an incredible cast and the diversity of original characters. It's really nice to see such a charming character as Lee Dong Wook's Grim Reaper, the main lead is also very good, then Kim Go Eun, Yook Sung Jae... And there are so many moments you can watch for a thousand of times and so few moments you prefer to skip... So, undoubtedly 10 ;)