I want to say that I loved this drama and watched it all in one sitting. I also had two holdbacks that kept slapping me across the face as the show went on because they never went away.
How they treated talk of suicide and used gayness as slap-stick.
Spoilers from here on.

Eun Yu travels back in time to literally kill herself. She tells Eun Gyeol multiple times, and NOT THING HAPPENS. HE BARELY REACTS AND BY EPISODE 12 IT'S LIKE SHE'S TALKING ABOUT THE WEATHER. IF THIS WAS NOT FICTION (AND EVEN IN FICTION), WHEN SOMEONE TELLS YOU THEY'RE GOING TO KILL THEMSELVES, COUNT YOURSELF LUCKY THEY TOLD YOU THIS TIME. I DON'T CARE IF THEY'RE JOKING, OR UNSURE, OR IT'S SPUR OF THE MOMENT.
THIS. IS. A. CRY. FOR. HELP.
I would give anything for him to have reacted appropriately once. To have acknowledged what she said, to have even checked in, "so, I hear you want to kill yourself. You serious?" Instead, by episode 12, she's holding full conversations about coming here to kill herself multiple times in an episode, and .... and.... HOW CAN THE CONVERSATION JUST CONTINUE. I'm sitting here, like, wait, dude, bro, like what? Are you at least going to ask her if she still plans to kill herself? She's talking in the present tense here and it's not the subtitles' fault.
If you were someone who has a suicidal friend or family member and saw this and even considered for a moment, "this is how people respond when someone tells you they're trying to kill themselves?" that's bad. If suicide isn't talked about in a community, and more media than not treats it as unreal, un-urgent, or even a joke... this will affect your view and behavior. You are also going to treat it that way. And if you, the reader are saying "media doesn't affect your behavior; no one's that stupid," have you seen the internet? Are you not here on this website? Have you heard of Hallyu? The Korean Government knows how much media affects people's beliefs and behaviours. They built their entire trillion-dollar economy on it.
If you are suicidal and see someone else's suicidal behavior talked about like this, you're going to be horrified. You're going to take this as more evidence that if you tell people they won't take you seriously. Also, this is Korea, one of the countries with the highest suicide rate in the world. If you think about it, it's absolutely f-cked up that they treat suicide like this. "Oh, suicide, it's a part of life. Let's pretend she didn't say anything. Everyone's a bit suicidal these days, etc."
For - using gayness as a joke was very off-putting. Each time someone accused the brother or father of being gay for the other, they both reacted like it was the end of the world (Maybe that's what the ultra-devout Christians want us to believe, so what, you want to forward their agenda) You couldn't just leave it at a friend being overprotective of a friend/family member they are afraid for? You had to say that that much affection and protectiveness between or shown by a man must mean he's gay!?!?! Wow. Just Wow. I know it's a plot device, but please. This is only a stairway landing up from killing the gays - yay, we made it into the 20th century, now let's just make fun of gay people and use them to excuse away too much male sentimentality. wow.