I enjoyed watching Start-Up overall but I do have some issues with it.
First of, great cast and acting. The show had a strong start and it had me hooked from the first episode. However, they had amazing characters but never really used them or pushed them in the background.

*SPOILERS*

My main issue:
The main storyline is Dal-mi and her first love, being Han Ji Pyeong, It frustrates me that she never really talked to him about the letters. There was a very short scene but after she had already chosen Nam Do San. They should have done much more with Han Ji Pyeong's character, instead of pushing him into the background.

Han Ji Pyeong would have made more sense for Dal Mi but they hardly interacted at all, besides their mentor-trainee relationship. Almost all the romance-screen time went to Nam Do San, even though Dal Mi could never state what she liked about him. Big hands aside...

In Jae is another character that should have received much more screen time. Now she just served as a reason for Dal Mi to find her lettter-pal, and afterwards In Jae is a 'savior' by hiring her younger sister.