What kdramas should I NOT watch?

I second heirs as one of the worst dramas I've seen, but as much as I loathe it I still hesitate in telling others to avoid it. I enjoyed a lot of the banter between characters & liked kang min hyuk's & krystal's characters the best, but it's a painfully stupid, frustrating script to sit through. it serves as an ultimate waste of talent, imo, & I didn't feel chemistry between lee min ho & park shin hye---that's mainly why I don't personally rec heirs, but for every person who hates it there is another who is loves it. ¯\_(ツ)_/¯

other dramas:
love cells: episodes are only around 7 - 10 minutes long, on average, so why did each episode feel like it took centuries to finish? I was so, so bored.
playful kiss: I don't understand wanting to center your world around one person, & that's exactly what the female lead does. there is a big disconnect between myself & oh ha ni, but it's doubly disappointing to me that she never once feels like her own person. where her crush goes, she goes; what her crush likes, she likes. I can't stand watching that! adding, too, that baek seung jo is the typical a-hole male lead, but what stood out as I watched the drama progress is that I saw no character growth in him (or ha ni, for that matter). he remains an undesirable jerk, so what she sees in him & why she continues to follow him around is a mystery. I don't get this drama at all.
pretty boy: boring, boring, boring, boring. boring. that's all I have to say about it.
sassy go go: there's actually a number of positive & enjoyable aspects to this show, but my frustration toward the script overshadows all of that. this one had potential but it starts going downhill when chae soo bin's character takes a huge step backward & ji soo becomes a second lead to his bff. it results in some pretty serious issues & character growth hitting the back burner, if not just stiffed, in favor of a cliche love triangle. sgg is overall disappointing & maddening.
school 2015: love the cast; hate the script. hate it. to date it's the most frustrating show I've seen. people will say what they will over the male leads, but that's just one out of many, many problems.
trot lovers: another boring drama, imo. the only strong memory I have of trot lovers---besides wishing it was over---is watching lee se young's character fall on her face. her being such a nasty brat made her fall the episode highlight, &, apparently, of the whole drama, because that is as exciting as it got for me.
she was pretty: I wouldn't say not to watch it, but it pretty much ends at episode 12. boredom reigns every second beyond that point, unless plotless fluff entertains you.

dropped dramas I don't care to finish:
monstar: every time I tuned in I hoped that this would be the ep our motley crew of misfits would get their act together... do they ever get around to it? I don't know & I don't care. the love triangle is just as dragged out & all the more reason I couldn't stand to finish.
super daddy yeol: I'll just say this: there is a difference between characters doing & saying dumb things for what they believe are right reasons (but still providing a decent/good script) vs. poor writing. I had three eps to go on this one but it felt like torture & I couldn't take it any longer.
yong pal: I tend to avoid medical shows because of my background & work but it's über ridiculous, point blank.
big: there came a point where I felt like big was stagnating, didn't know what to do with itself, & was about to spiral into a bigger, stickier mess. I know very well how it ended, & I'm glad I walked out when I did.

I think that's all from me...! it's nice to know which dramas might be bad enough to weed out from your ptw list, but everything I said & don't recommend is based solely on my preferences. that's why I always say to at least try something to be certain about whether or not it's worth turning your back on.

K-dramas I somehow managed to finish although I don't know how. I probably slept while playing the video...:
Yong Pal: I usually enjoy watching medical dramas and I liked it in the beginning but then the plot holes got more and more into my focus and it got super draggy and ridiculous
Orange Marmalade: In the beginning it was nice as there wasn't much tiptoeing around their crush but it was so so rushed and at ep 3 or so they suddenly made it a sageuk/historical drama which seemed so out of place and was boring. The female lead is absolutely not a good actress but you could see a little improvement towards the end. Yeo Jin Goo is one of my favorite child actors (although he is no child anymore) but this was probably his worst drama.
The Heirs: It was all hype because of the cast. There were nice moments that made me laugh but they were all scenes concerning the side characters which were good here and there but the writing and the main couple made me cringe and angered me as hell. Lee Min Ho's and Park Shin Hye's acting was HORRIBLE and I just kept wondering why they chose to take on roles of highschool students although it's so obvious that they are way too old for that and although they had a lot of older before The Heirs.
Nail Shop Paris: I was so confused about that. The writers seemed to want everything but got nothing in the end because everything was so messy. There was no chemistry and you couldn't care less about the characters.
Cheese In The Trap: This was apparently also hyped and loved a lot but I couldn't get it. The plot could have been so interesting but it was not at all. I didn't care about the characters except for the 2nd male lead. There was no character development whatsoever and the female lead made me furious because of her frustrating descision-making.

Dramas I couldn't even finish:
Highschool-Love On: I don't even know what happened. It all got so draggy and ridiculous.

@oliviadixon "u  guys arent romantic at all how can ppl say to the beautiful you the heirs boys over flowers coffee prince scholar who walks the night playful kiss those are classic romantic movies"

I agree that coffee prince and scholar who walks the night are pretty romantic.  I'll disagree with Heirs and Playful Kiss but I want to discuss the Heirs.
I'll also disagree that I'm not romantic. My three favourite dramas,  Faith, Healer and Oh My Ghost are my three favourite dramas because of the way the characters and their relationships are depicted.  All three of these relationships include: respect, love, good communication.  mostly these are shown through the actions of the characters, not just empty words 

The Heirs boring and drawn out.  Kim Tan is abusive to pretty much everyone.  he ruins lives without thought because he is entitled and privileged.  everyone is his plaything.  The only character who shows any personal growth at all is Choi Young Do.

Quite often when i watch a really romantic drama the kisses are excellent.  The kiss in the store room in the Heirs is not a kiss, it's a sexual assault.

I can't really think of anything nice to say about this drama except Park Shin Hye looks good when she cries (which is a good thing because she does a lot of it due to her emotionally manipulative and abusive boyfriend)  and I got a lot of laughs from Kim Tan's ridiculous outfits.

Here's an article that explains (way better than I can) why Kim Tan is so problematic, there's probably spoilers:  KDrama Fighting

I agree don't watch To the Beautiful You! I didn't like it! I don't think Sulli played the role well...I totally prefer the Japanese Version of it (Hana Kimi). Maki Horikita did a good job with the role of the main female lead.

"Local Hero"
Was a total waste of time... -_-

As much as I really like Park Shin Hye, it seems like most of my entries are dramas starring her... >.<

Avoid
Flower Boy Next Door: I think it just didn't start off good and wasn't enough for me to stay and watch so I gave up after the first ep or two.
Heartstrings is another one. I only watched it because my friend was at the time and I couldn't understand why she liked it? The plot seemed so plain and boring.
Cheese in the Trap: the plot was really weird and I just didn't understand it...
Playful Kiss: not sure if it was the acting or...? I just didn't feel the chemistry. This was a disappointing remake because I really liked the TW version.

Proceed with Caution
The Heirs: personally I remember I really enjoyed it when I was watching it but if you ask me what it was about or what I liked about it now, I can't answer that. I honestly don't remember anything about it besides the fact that it was the typical "chaebol guy likes poor girl and they're broken up" plot.
Doctors: the story was very interesting until they proceeded to when PSH became a doctor. From that point on, the plot became slow and draggy. I think the only reason I've been able to keep watching is because it's a medical drama... I don't feel any chemistry between the two leads, though :/

The heirs -  Overrated. It was horrific and a pain to watch. If it wasn't for my whole "try your best to complete a drama you started even if it isn't that great" thing, I would've dropped it long time ago. What's great about it is the awesome cast. I mean obviously, if I was directing a drama that's boring. unoriginal, and predictable, I would put as many famous good actors as possible too. With Lee Minho, Kim Woobin, Park Shinhye, Choi Jinhyuk, Kang Haneul, Krystal, Kang Minhyuk, Kim Jiwon, and even a Vixx cameo, obviously the drama will get overrated and gain so much publicity. I guess that was the best part of it. Seeing so many actors I like all together. Other that, it isn't worth your time at all. 

Cheongdamdong Alice - An even bigger pain to watch.Terrible plot and the most annoying female lead ever. What kept me going for a while was the perfect male lead character who was perfectly played by Park Shihoo. If it wasn't for him, I wouldn't have even been able to stand watching one episode. There's honestly NOTHING about the female lead that's likeable. She's a gold digger, she's incredibly stupid, and she doesn't deserve Park Shihoo's character at ALL. 

Pretty boy/Bel Ami - Dreadful. Possibly the worst drama I watched along with Cheongdamdong Alice. Terrible plot. Terrible everything. 

Big - Lol. If I hadn't been 13 when I watched this, I wouldn't have watched this. It's only the actors which would allow me to be able to stand it now.

High Society - HORRENDOUS. The main couple hardly get ANY scenes together, while they focus on the second lead couple where the female is incredibly annoying and all I feel like doing is shutting her up. Also, another gold digger scenario, just from the male's side this time. Ugh.

My Fair Lady - Booooooriiiiiing. Yet another gold digger scenario. What's up with these gold digger plot ideas? Do people actually believe its a good idea to make one of the leads a gold digger? The second lead syndrome was strong here. Song Joongki's cameo made me excited enough to keep going lol.

Pinocchio - Another drama which, like Heirs, is highly overrated. Don't get me wrong, I love Lee Jongsuk. And the actors did a really great job. But the plot was meh. And it didn't have anything special about it. 

Fated To Love You - After watching the Taiwanese version which I really enjoyed, the Korean version was unbearable to watch. I couldn't even get through the first episode. The humour isn't even funny. The whole drama is cringe worthy. The plot is great though, but they ruined it. If you want to watch a drama with this plot, watch the Taiwanese version, where the humour is actually funny and the drama is worth your time. 

Dots
Mary Stayed Out all Night
Pretty Boy
Wanted
Awl
Local Hero