For the sake of reaching (and saving) as many people as possible - this review will be spoiler-free, and shouldn't contain anything that isn't already in the above summary of this drama.

I was told this drama was very good. I was under the impression it could easily be a "staple" drama that you need to watch if you wanna be "in the know" about all the good K-dramas out there. Sadly, this is far from the truth.

Starting with the good (because the amount of 'bad' is far greater) - the PREMISE of the story is very, very good. The set-up had a lot of potential, and you can see pieces of a good storyline scattered throughout the badly-executed series. I found myself wanting to know more about how life and death worked in this universe, about the mysteries of the world and the mysteries of the characters' pasts. The framework is there, very solidly - it's just a shame they didn't put it to good use.
I will also say, the character dynamics (save for the negatives which I will mention in a moment) can be very heartwarming and funny, and the casting certainly wasn't bad.

But now we move to the bad.

To begin - I know most people mention the age gap, but if I am to be honest, the ages themselves would've been somewhat forgivable if it weren't for the writing making that gap seem even larger. She addresses him as "mister" (or "ahjussi") throughout the series, she never stands up to him no matter how he treats her (feeling similar to a daughter respecting her father), and while she's giggling and wrinkling her nose and running around like an innocent child, he's either standing by like an obligatory parental guardian who's watching over an excited kid - or he's admiring her innocence, which on its own isn't bad, but when compounded with everything else becomes creepy. Their relationship feels much more father-daughter than romantic.

And something I'm not hearing many people talk about - for a good while, their relationship begins as something very unhealthy. He's selfish, he treats her with disrespect, and she lets him do so. At first it felt like something that would go away - but instead, they just eventually get cuter out of the blue. It's like the writer wanted to write some kind of "slowly developing relationship" thing - but it didn't work out, and instead it's just hard to watch and a little confusing. I wasn't able to root for them for a majority of the series.

On the other hand, the supporting roles have a MUCH better, deeper relationship, and they themselves are deeper characters. In fact, the side characters themselves are more interesting in general. They undergo more character development throughout the series and have a lot more depth than the main characters do - and one character who should have played a more important role, certainly never did.

The list of "negatives" does keep going: The pacing becomes extremely slow after the first five-or-so episodes, there are many inconsistencies (some very blatant especially toward the end), important plot points end up feeling unimportant, there are unanswered questions that probably should've been answered, there's a point where a character makes such an obviously bad choice that it's practically unbelievable, and in spite of being 900, the Goblin tends to feel more dense than a 20-year-old. (Seriously, at the age of 900 can't he at LEAST be given the intelligence of an 80-year-old?)... and I can go on. In detail.

I wanted this to be good. It has SUCH an interesting setup, but it just isn't. Even if half of these flaws didn't exist - with slow pacing and a semi-repetitive nature, it still gets very bland. When you root for the side characters and stop caring about the main characters, I'd say that's when a drama has ultimately failed.