It feels unfinished, in more senses than one.
I won't blame the actors. At the end of the day, I think they did what they could, considering how POORLY WRITTEN the story was. In fact, the script and the plot itself are the real problems here. The story lacks importance. Big issues were taken lightly, while stupid things were taken out of proportion.
Let's review the main issues:
1) We are presented with a love story about two people from apparently two different worlds. However, in 24 episodes we never knew about those worlds. We never got to know their families, so the 3 or 4 times that any of the leads received a call from a familiar... it ment nothing. In fact, it felf random.
2) If the little girl and the little boy were around the same age in the flashbacks, how on earth did he become 4 or 5 years older in present day? This issue has nothing to do with the "real" age gap between the leads (I'm used to 30-something people playing highschoolers, so I really couldn't care less about that). I'm talking plot wise. He's a MBA student while she's just starting architecture.
3) The heart-shaped music box did not get the importance that it should have. It was supposed to mean something, a real connection or a symbol... at the end, the fact that they met each other as kids ment nothing. A wasted oportunity.
4) Stupid situations taken out of proportion: (A) Nobody spends two days in the hospital for a simple gut punch. (B) The whole "she went missing" situation was cringe. The classmates did nothing, but then comes the heroe and instanly finds her... unharmed,.. sitting there,.. waiting. Not even a scratch!
5) Abuse of flashbacks. Background music louder than dialogue. Some scenes felt out of place.

Anyways, it wasn't good, but I've seen worse.
Despite everything, there was some good chemistry between the leads (I mean, Dennis could have good chemistry with a plant after all), but the dialogue was really bad. So, every sweet moment between them was hijacked by some bad wording statement.