Loved absolutely everything about this one. In no particular order:
1. Superb acting -- could have expected nothing else from the leads, but the supporting cast were so stupendously wonderful that one can sense the depth in them -- it really does feel like they have lives outside of the show.
2. Wonderful music -- suspenseful yet not intrusive (and main theme is very catchy!).
3. Loved, loved, loved the character interactions -- It felt like the characters had real relationships (acting definitely contributed to it, but so did the writing, editing, and direction). And thank you, thank you, thank you for not forcing a romance in there.
4. Focus on the work setting -- I know this is a crime drama and the mental puzzle is what is meant to keep it going, but to me, the little interactions between colleagues is what truly makes the show outstanding. It feels like the writers may have actually worked in real life, know what work is like, and have managed to make those real-world experiences entertaining. Never having worked for the police or prosecution I cannot comment about how accurate the actual work responsibilities are (and have a feeling that many of the male lead's activities would be highly inappropriate or at least unlikely for a prosecutor), but at least the situations feel organic.
5. The story -- although there are some minor plot holes and not all deductions are completely justified, the story nonetheless makes sense, and it is clear on rewatching (which, yes, I have done a few times) that the story had been meticulously planned.
Not only one of the best Korean shows that I have watched, but one of the best on Netflix right now, IMHO.
Can't wait for the next season!