==Breakdown==
Background plot- 35%
Romance - 40%
Misc comedy - 25%

Background plot - Excellent
Romantic chemistry - Excellent
Misc comedy - Excellent
Production value and direction - Excellent

First, the script. The script and story are brilliant. ABSOLUTELY BRILLIANT. There are a few people who point out the age difference between the main characters. This didn't bother me for a few reasons:
1) There's no romantic chemistry between them until she's older, but I loved the bickering friendship chemistry between them in the early episodes. And her young age at the beginning contributes to the absurdity and therefore comedy. You're not supposed to see adult love between the characters (at the beginning). You see a bickering, frustration, growing respect, empathy, eventual admiration and like between two humans, which is appropriate.
2) The main premise of the story is he's a hundreds-year-old Goblin; you're going to have to be willing to suspend disbelief, people! If the part of the story you're hung up on is the "age difference" between a 200+ year old Goblin and the "bride," then that's odd.
3) His character, honestly, wasn't very mature. LOL! I mean, he was old, but his maturity level wasn't commensurate with his age. He seemed petty and kinda tired of life when the show started. He continued to be petty (but less tired) as the show went on. The character was stuck (mentally and emotionally) at the age of his "death," likely 30 or so. That was made very clear. Thus, she grew up and so did he, eventually coming together when they were similar in mental and emotional age.

Second, third, fourth . . . one hundredth, the cast. Holy moly, there has never been a more perfect cast. My Drama List should allow users to apply a "11 star" option once. If they did, this show would've received it from me for its casting. Obviously, all the main leads were incredible. This was my first introduction to both Kim Go Eun and Gong Yoo, and I have become a big fan of both. Their chemistry with each other was incredible. Their chemistry with other characters was also incredible. The way they look at each other in later episodes (just look) will give you butterflies. AAAHHH!!

The music must be mentioned. This is the first (and only) drama where I still listen to the entire soundtrack. Each song is worth listening to on its own. 10+++ stars for the music. It has become the drama against which all other OSTs are judged.

I want to go back to the script for a second. I can't say enough about how great the dialogue was. Obviously, the actors' delivery was essential to bringing it all to life, but some lines / interactions were so brilliant. Funny, poignant, witty, engaging. So good.

Direction was good. Art direction, cinematography, and production values were also good.

Overall brilliant. Loved it. Have watched it 2x so far. I'll probably watch it 10 more times at least.