I had put off watching Goblin for a long time as I knew it's epic so I needed a devoted time to watch it till the end.
Goblins becomes the highest standard of my reviews on dramas I completed (mostly for Kdramas).
What I learned from this drama: forgiveness (past events of Goblin/Grim Reaper/Sunny) and that there is timing for everything you encounter in life (Goblin waits for 900+ years for a 19 year old bride to show up in her first life)

Tons of previous reviews posted already, so I will just point out what I like about this drama:
Story: No boring scene, no skipping for me who likes skipping scenes. Side stories of dead people and ghosts are also meaningful and touching.
Directing: Just perfect, won't say much, check out other Director Lee Eung Bok's cinematic work
Editing: Details are woven perfectly, and all related. Pieces of flashback are not boring.
Screenwriting: witty, deep, poetic. Follow Writer Kim Eun Sook for other high quality dramas.
CGI: Battle, fight, and snow scenes of Goblin are epic
Location: Quebec Canada, can't have other perfect location than fall season with French touch
Make up: Can transform one actor/actress to different roles. For examplem Kim Byung Chul as Emperor advisor, modern secretary, ghost, and cameo CEO.
Costume: Classy and stylish. For example: 19 year old Eun Tak does not look poor and childish. Personality symbols thru costumes, for example Shamsin Lady's red dress, Sunny's bright colored clothing, and Eun Tak's red scarf
Cast: Perfect selection for every main roles and supporting casts are not bad either. Gong Yoo and Lee Dong Wook supernatural roles don't need to talk much but their facial expression shows deep communications. Kim Go Eun's natural beauty fits for 19 year old girl, Yoo In Na's cheerfulness matches Sunny lively personality.
Acting: Lots of scenes with no dialogues just facial expression and physical movements speak louder than many dialogues
Music: songs are created and picked perfectly support the popularity of the drama and songs themselves
Rewatch: perfect, but I will wait for a while until I can spend a large portion of my time to watch it