This will be a complete spoiler rant, so if you don't want to know what happens in this show, stop reading. It's been a long time since I have been so frustrated an angry while watching a drama.
1) First and foremost, I hated the FL character. This woman was actually stupid. I mean, how many times are you gonna stand in the street and wait to be hit by a car?! Why would you meet up with your cheating, scumbag of an immature ex-boyfriend? Why would you let the girl that slept with your boyfriend boss you around? You're interested in a guy, and he tells you that he's approached you because he thinks you killed his brother, and you don't get mad? Not only that, you tell him over and over again that you like him, knowing that he just thinks you're a killer? You find out who one of the killers is, and you continue to hang around him? AND, you accuse him to his face?!?! You go wandering around at night, by yourself, to find information on the killing, you run into one of the killers, yet you continue to wander by yourself? Then, there is the total lack of work ethic and the fact that she seems unable to do the simplest jobs correctly.
2) I didn't particularly care for anyone in this drama. The ML, the looks he was always giving made me think that he was not the best guy in the world, either. Were we supposed to think that? Why?
3) Too many episodes. The red herrings were flying fast and furiously and there were almost complete episodes that I fast forwarded through because the flashbacks were completely repetitive, and they kept trying to make us think things that weren't true. This can work well, but in this case, I felt like the writers were writing the script on the fly and had no idea where they were going to take the characters.
4) Lack of chemistry between lead characters. They actually seemed to be uncomfortable with each other. It was painful to watch.
5) Once I finally found out what actually happened, it was such a letdown. Again, I felt like the writers boxed themselves into a corner and wrote and ending that no one would care about after all this time.
6) Time Jump - I know, it's obligatory in K-drama world. He left without speaking to her, leaving her a note. Comes back a year later like nothing had happened and she just forgave him?!?! I'm not one for playing games in relationships, but I don't think anyone in their right mind would forgive and forget that easily. Plus the fact that the chemistry was still lacking. This was the first time I've watched a drama where I would have been perfectly happy if he never came back.

The music was the highlight of this drama.

I would not watch this drama again, even if someone offered to pay me to do so.