For me too. The fact that they are 5 friends consisting of 1 girl 4 boys reminded me Reply 1988, first. But also the way they treat each other is also similar, so it makes me happy. Yet each of them has different personality, so I can't say this character resembles that from Reply 1988 etc.  Lastly, Jung Won's mom and her friend's (hospital director) friendship reminds me Reply 1988 too.

Btw, in one of the episodes, Taek's dad and Sun Woo's mom were guests, they portrayed a couple in the hospital. I think, they played their characters from Reply 1988 because Taek's dad was clumsy again (like when he collapsed and hospitalized and could not take care of himself in Reply 1988).  It felt really good. Also in the first episode, Deok Sun's dad and Jung Hwan's dad played priest brothers of Jung Won. It was also splendid. 

The stuffed toy that Prof Ahn Jeongwon got from the bank. I saw a theory regarding that one. Then I suddenly remembered the stuffed toys that gave us hints (that I didn't notice) on Reply 1988.