Pros:
Loved the cast
Liked the soundtrack
Found the premise interesting

Cons:
Plot holes
Sometimes disjointed editing / scene jumping
Adding new questions without answering the old questions
Adding new characters at the end
Leaving too much to a potential S2, instead of a safe 99% conclusion with a slight hint of potential for more

I have been disappointed in the finales in Kdramas the majority of the time for years now. I can usually handle it as a mild frustration. This one left me shaking my head in wonder and for a while, I had no words…

I hope the director learned that you have got to provide your audience with actual answers that they have been faithfully watching for weeks and waiting for.