A drama that had great potential but turned out bad

It's a very difficult question, I know, because sometimes a drama can be good for ones but a disaster to other. But let's talk generally, what drama do you think had potential but was bad?

Mad Dog: amazing cast, great plot on paper but was just lacking something

W: amazing first half but went downhill from there

Bad Guys: the plot interested me but it was just average

Love Shuffle: interesting concept, great screenwriter, didn't like the drama itself

Ame ga Furu to Kimi wa Yasashii: sounded like a good drama and I like the screenwriter but hate the actress and found it uninterested

Kono yo no Hate: see above, minus the actress

Honestly I know people are going to dislike this answer but Pride was awesome til episode 7, after reading the episode synopsis for the following 4 episodes I actually had to build up the mental strength to finish it, and then forwarded through most of it.  

Amazing story of friendship blooming into romance. It could have been great if it was the usual 2 episode hiccup every drama gets but this one ran from eps 7-11.

I have to complete a drama once I start it, I have only ever dropped 3 dramas and I just could not force my self to watch them any more.

1. Dramas written by Jang HyeokRin due to not being abke to sustain a very solid narrative: Reset, Yongpal and The K2

2. Reply 1988 - i love everything about it, except how yhey treated the character junghwan

3. W: 2 worlds - it was super exciting at the start but it lost its way toward the end

4. Cheese in a trap - such an awesome drama but i guess since the manhwa wasnt finished when the drama aired, it lost its foorting and direction

5. Gods gift: 14 days - the ending, urghhhh

6. Rush to the end of summer - i wish there was nore scene at the ending reunion

7. Lovers in paris - the ending was horrible

W: Two worlds - like others have already said, I think this one suffered from having such a spectacular and unique beginning that everyone's expectations were set too high and the drama just couldn't sustain it. Still worth watching, but yeah.

Yongpal - I just wanted to watch the story about the super cool rogue doctor from the beginning, not all the weathly family politics/manipulation it turned into.

Missing nine - great premise and great acting, but the script lost its way somewhere. And that ending....I will never get over it or understand it.

Hospital ship - the premise was great, but it's been awhile since I've watched something with so many tropes and plot hurdles written into such a small number of episodes at the end. The last episode specifically, just completely unnessecary. 

Literally every drama penned by Kim Eun Sook. She's a great & interesting world-builder, but the execution just isn't there every, single, time. But tbh, almost every drama is this for me. I can't even think of many individual examples because most start out fun (or at least have a fun premise) and turn out unwatchable. Youngpal is definitely an example, it had so much going for it: the character himself & Joo Won were really enough for jackpot, but virtually everything else is a total, total miss. Especially the romance, which just wasn't believable for me, at any stage, especially the too-quick progression? Speaking of romances, Sungkyunkwan Scandal's, imo, dropped the ball when it had the mc hook up and obsess over IMO the wrong dude, like it was still watchable because of the other interesting characters & parts of it, but the otp itself is a misfire (and, like, why would they make the actually obviously least talented male of the three leading ones the main? so weird). Omg, I just thought of the biggest example for me, probably, The Moon That Embraces The Sun. Holy shit. Don't get me wrong, I'd still watch it & it's somewhere in my vast top 30ish but maaaaaaaaaaaaannnnnn they had the opportunity to create a seriously amazing drama, IF the amazingness of the child cast continued, which it so, so achingly didn't. I just couldn't unsee the perfect representations of the kid characters to their lackluster adult counterparts: the whole time, I wanted to be watching the Kim YooJung Yeonwoo, the one with the air of wisdom & grace, and Yeo Jingoo Crown Prince, confident, adorable, in love & shattered. The adult world in comparison to the children's just felt small, and even with great "victory" at the end, it didn't feel like the victory of those whose side we were on, more like that of the imitators. Idk. Can u tell this bothers me lmfao? I basically wrote an essay.

I am actually quite tolerant to many plots but I really can't stand the ones below

Yong Pal: greatest disappointment- I thought there were more about the adventurous doctor and his gung-ho stories but got sucked in a real bad family politics and power struggle. And ya unbelievable romance too with a fairytale ending

W-two worlds : Great plot and idea but past halfway it was all over the place.  Amazing first half and after that just back and forth and unbelievable ending

Goblin- great start ,amazing idea but became too cheesy after ep ten when all the focus was on the love story between the main leads

Descendants of the sun: started out great , exciting then became plain cheesy love story and ended with unbelievable ending. How could he survived one whole year in captive  without any health complications and be at the same place back to  where they met?

Coffee prince:  disappointed as it turned to be all about Gong Yoo's character. The writer should have explored more on the main girl's character and more involvement of the coffee shop baristas . 

° Beautiful gong Shim (so good at the beginning, so boring after 10 ep)    °Secret Garden (for me 16 episodes were enough, 20 unnecessary)

° Suspicious partner  (no comment, total delusion)   °Man x man (I just could not stand HER even Park hae Jin was great)

°Pinocchio (I dropped at the last few ep)