1. Sam-Soon's situation after leaving Serizawa wasn't explained at all, the only thing we learn is that she rejected the proposal from the cheating dude, as mentioned by her friend, but no details regarding that. Being alone all that time before Serizawa got in touch with her again must have been very difficult for her but we don't get to see that apart from the very last page.
2. Serizawa's father situation got some attention towards the end, but it was never really shown what happened after he left his family and what kind of person his co-worker truly was.
3. Too little of interactions betweem Serizawa and Nana(the singer) were shown to justify her falling for him(that's just my take on it, could have been a love at first sight for what it's worth), also feel like she should have gotten more focus in that specific arc in general. Her getting back on track with song-writting was a nice detail though, considering she was told to write songs about heartbreak.
4. Serizawa's mother's past self was briefly mentioned/shown, but I feel like at least a flashback of how she was before she met her future husband and how she changed would have been interesting.