1. The cast was perfect, every single one of them especially Jun Ho and Won Jin Ah.
2. Good ending that finishes up the story well.
3. There is no perfect character, all the charcters were realistics each with their faults and strengths which is realistic and how characters should be.
4. Great soundtrack that fit scenes well and the show also used silence really well in key scenes which needed it.
5. cinematography was done really beautifully.
6. no cliche romance, no car crashes or amnesia etc. to build up tension.
7. it's daily life - nothing over the top, everything is realistic to even the smallest details like gang doo's phone and no stupid misunderstandings.
8. The plot was written really well and everything comes together perfectly.
One of the first dramas where there isn't a single thing which would only happen in drama land.