Obviously, a review a few years than useful to anybody, but here it is!
I searched far and wide for the show since there are so many references to it in various reviews for other shows, and it seemed like a worthwhile effort. I ended up binge watching it (which I really try not to do) -- it sucked me in.
But, to start with, the negatives:
* The age gap thing. It was physically gross. As others have pointed out, the female lead acts very much like a teenager, a young and immature teenager, and the scenes in which the two leads are pushed together for romantic reasons were truly gross. Which is actually a credit to the acting, since in later part, when the female lead is supposedly 10 years older, the feeling disappeared. Still -- it really did make me feel gross to have the romantic scenes between a teenager and an adult.
* I may be the only one, but I did not like the secondary romance -- it made next to no sense. The two actors were wonderful -- but the internal logic of their history was nonsensical.
* The ending. There are a couple of components to my dissatisfaction with it, and they all involve lack of any actual resolution. For one -- the goblin "dies" or whatever it is he does, but then is called back -- and is still a goblin. Wasn't he supposed to die? Does this mean he never dies? And the whole thing repeats three more times?
* The "reveal" of who the grim reaper is was telegraphed so early that it was not a reveal at all.
The positives are what others have pointed out: Engaging story that, overall, holds together logically; good acting; excellent chemistry among all of the actors (not just the romantic leads) -- the wonderful bromance!; the beautiful music.
So, in the end, I am glad that I got to watch it, despite the occasional grossness.