It's highly unusual that a drama would rile me up as much as Longing For You has. I had such high expectations after the first episode but it was a let down very soon into the show.

The Good:
The idea behind the plot was good and it started off as an interesting mystery with the serial killings.
The OST is great. I love the intro theme song.
Some of the beginning and later episodes are good but not enough to make up for places it lacks.
Some of the loose ends are tied.

The Awful:
Predictable - by the 4th episode, you will know exactly what has happened. The rest of the episodes is just the rest of the characters being oblivious and not able to guess anything and it is beyond frustrating to watch them run around like headless chickens.
Poor acting - Usually I can blame the writing for ruining a show. But not here. The actors have done such an unconvincing job at every stage. I felt like not a single person cared for Jin Woo. Not his brother, not his mother, not his adoptive mother, not his friend, Young Joo. He had no one! Even towards the end, I didn't feel like anyone cared in the courtroom. Nor when they were first watching the videos. A normal mother or family member would cry and scream in such types of situations. They would be out of control. Nope. Not this family. They, at best, could muster a teary-eyed shocked look on their face. That was the most heart-breaking thing. I've seen extras in other dramas react more than this group. I, in fact, was feeling more emotions than those family members.
Humanizing the villains - This was beyond annoying. Making killers and psychopaths "look" remorseful. People can't just take lives without a thought if they were humane to begin with. It's not like they were acting in self-defense. The writers should have let those characters stay villains. You made them look pathetic by the end. And some were doing comedy. Really? And Dr. Choo gets no punishment after what he did? How can the family even trust this man, let alone be normal with him?
Romance - well, there is no romance. Also, there is no chemistry between the two people who apparently like each other. When they confess their feelings, I went, "huh? what?". I couldn't believe it. This is a very minor point though. I don't care if there was romance or not. But if you are writing it into the plot, at least make it believable.
Convenient evidence and confessions - again, this is a minor point since most shows that want to take down villains end up finding all the evidence necessary. It was just the brazenly tight-lipped murderers all decided to confess one by one. The evidence that was supposedly non-admissible was suddenly allowed in the courtroom. At least stay consistent with your own arguments or explain why they were allowed in court.

One should usually be satisfied when things are wrapped up but looking at everyone joking around and happy till the end, I felt like beating those characters up for never once fully mourning when they should have. They frustrated me throughout the series.

I would like to forget I ever saw this show with how disappointed I feel.