The biggest problem with these 1 season, serial K dramas is maintaing consistent quality of episodes throughout the run.... many shows start out great for the first half.... but then really go off the rails into draggy plots or nonsense. Sometimes it's just a few episodes of slowdown, other times the show completely tanks after a certain point and I lose all interest, even if it started out great. To me, this doesn't mean the show is complete failure... if A show gives me 8-10 hours of entertainment before tanking, I can still look back happy to have spent the time with it.
What shows have you experienced this with the worst?
The Moon That Embraces The Sun is the definitive answer, imo. (There are tons I'd like to get into when I can actually think of some, but this is THE one that I was most let down by tbh). The kid cast just set SUCH a high bar that the adults never actually came close to, everything just felt slightly off afterwards, from the world itself feeling *smaller*, to Han Ga In, who I truly love but just did not possess the same obvious queen-like grace than Kim Yoojung did, to the main couple who were lacking that....something. The child-actor episodes are pitch perfect and so affecting, the grown up-actors felt depressing, theatrical and a bit mannered/contrived. This sounds like I hate the latter part of the drama: I don't and it's certainly watchable on its own, but it is absolutely not as good as the first five-ish episodes. Like, the difference is glaring.
Right now I'm actually watching a daily drama (lmfao) (if anyone cares it's Apgujeong Midnight Sun) and it reminds me of something I soooooooo dislike and happens so often in dramas in general: when the main couple just miss the time they SHOULD'VE gotten together and drag out the inevitable for so long that it's no longer anywhere near as affecting or interesting, and no one in the drama even cares all that much about it. Like, you can make a good drama with the OTP paired up. If the entire conflict resides in them being apart, that's simply lazy writing (unless it's literally tied to the plot, and one of the only instances I can think of where that is a valid excuse is My Beautiful Bride) like audiences WILL watch shows even when the couple does get together, they just need to be written so they aren't deathly boring.
Marine: that's par for the course with daily dramas. Things that could happen in a single episode will be dragged out over 10 episodes to fill time. I see it in pretty much all 100+ episode daily dramas.
Doctors (pretty solid first couple episodes, but got hella repetitive)
Gu Family Book (had potential; the parents' story was way better)
Hit the Top (great until we know what the final ship is)
Marriage, Not Dating (hilarious first half; dragged the second half)
Suspicious Partner (great start, but the romance was too repetitive. but Ji Chang Wook is always a good thing, right? xDD)
The Guardians/Lookout (lit start, but gets dumb and dumber as it continues)
Uncontrollably Fond (solid melo start, but gets messy)
W - Two Worlds (okay start, but got too twisty for its own good)
Witch's Court (awesome first four episodes, then turns into a pot of makjang)
While You Were Sleeping (repetitive after the first few episodes)
I came here to say what you said about Doctors. The FL was fantastic in the beginning imo, but as the drama went on she became less interesting and her relationships with just about every character except the ML were more interesting than their relationship.