Rooftop Prince - I thought it started out promising and the comedy bits were gold, but the latter half got bogged down by the boring company plot. I just didn't understand why everyone seemed to think it was so great.

Kim Sam Soon - I just cannot understand the popularity. I could not get into this show at all. I don't care for the characters or the plot. Maybe it's just because it's an older drama? I'm not sure, but I ended up dropping it.

Faith - Every episode I kept hoping the show would improve but nope. I finally dropped it after 18 episodes. The only reason I watched for that long was because of the side plot with the king and queen. I suspect most of its popularity is just because of Lee Min Ho though.

Dream High - I enjoyed it to a degree, but a lot of the time I found it rather boring. I did love most of the characters though.

My Girlfriend is a Gumiho - I have this drama on hold because I have so much trouble keeping interest in it. I honestly don't understand why I can't get into this one because I normally love supernatural type stuff and I love both of the leads. For some reason it just doesn't click with me.

Of course, these are all just my opinions and I know that everyone has different tastes.
Healer - Bland and uninteresting mostly, and I had no interest in any of the lead actors nor characters.

Faith - Average with nothing that really happened throughout the drama.  Lee Min Ho seemed a bit blank in this as well, so was difficult to like his character or have any believability with the romance, although Kim Hee Sun was great in her role, and the only reason I kept with it was because of Ryu Deok Hwan.

Kill Me Heal Me - The majority of the storylines introduced too soon, so nothing really to build up any interest, also predicatable mystery and over-reliance on repeating the same past incident over and over, and above all the comedy screaming particularly from the lead female was really irritating.  Ji Sung was great with all the personalities though.

Nine Times Time Travel - I usually like time travel dramas, and I like a bit of melodrama and the occasional sad story, but sadly this series just went into the realms of total unecessarily depressing stuff particularly from halfway through onwards.

The Master's Sun - As much as I like So Ji Sub, I didn't like his character in this, and whilst it wasn't bad, I generally just found it to be an average drama.
Off the top of my head, two come to mind (there  are others, though):  My Girlfriend is a Gumiho (I just couldn't get into it), and A Gentleman's Dignity.  The latter I thought treated women badly, and I just couldn't get past it.  
Heirs is definitely overrated, though I did enjoy all the supporting roles.

currently watching sungkyunkwan scandal after reading loads of promising reviews online but on episode 11 now and I'm not too fussed on it..
She's so Lovable
You're Beautiful
Doctor Stranger 
My Princess
City Hunter 
BIG
K-POP - The Ultimate Audition
Dream High
My Girlfriend is a Gumiho .....
Sorry for the bad english, I am not native of a country that speaks english, and I have never been to a country that does, so ignore it. 
Coffee Prince: I could not get myself to watch it, two episodes were more than enough, I never jump parts because I feel like I am loosing something important, but in those two episodes, I jumped many scenes, so fucking boring. 
My Love From Another Star: It was good in the middle, from the 3 to the 16, but I really hated the beggining and the end, the main couple was so bleh, and I couldn't bear the main girl, she was the worst. I only liked the middle because of the villain (he was amazing) and Park Hae Jin. 
Healer: I did like it, but all the characters (but healer) were too bothersome for me to watch it, and the parts with their past were boring too. Not that good of a korean drama. 
heirs (and any other lee Minho drama), secret garden, my love from the star,... imo they are sooo exaggerated! everywhere you look TV, websites, posters, they're everywhere, and also Korea's slight obsession with won bin, its creepy af, they idolize people way too much..
The Heirs 
I didn't like the 2 main characters, I couldn't see their chemistry between each other... I liked the second couple with Minhyuk and Krystal, and I think Kim Woo Bin acted great even though that I didn't like his character. I think the drama was so cold and the end was... ehm... I'm not gonna tell so I don't spoil anything...

Playful Kiss
I just couldn't stand the girl main character because she was so embarrassing in a bad way... ok, there were some funny scenes when she was like that, but it was boring and painful to watch most of the time... I almost cried inside because it was so bad for me - and I'm the typical person who "must" watch the whole drama even if it's bad because I want to see how it all ends etc. 
Secret garden, Marriage not dating, BIG , Inspiring  generation  and Gu Familly Book.
Sooo much hype about your beautiful.  It was like pulling teeth to finish this drama sorry no offense just my opinion. The female lead just had no backbone.  I  came to the conclusion it was all about the eye candy. My second  that just had me in  awe was My girlfriend  is a Gumiho.  I almost didn't even finish.  Ok.kinda got the point of the drama but it was literally down to the last 2 or 3 episodes. I  think I was daydreaming about other dramas during the whole middle of this drama. It didn't keep me engaged. 
All of Lee Min Ho's dramas
All of Hyun Bin's dramas (especially Secret Garden and Kim Sam Soon)
Flower Boy Next Door- the best part was Kim Seul Gi and Enriques cuteness!
Secret- dont get me wrong I loved this drama but honestly I dont think it was anything great or special... it has the same melodramatic cliches as most dramas imo! And if this really happened in reality I would probably hate Ji Sungs character lol
Rich Man Poor Woman
Master's Sun
Most people are right. Boys Over Flowers is so overrated. I have no idea why anyone likes it.  One of the worst is The Heirs. They pushed that drama so hard but it was awful. And it was so boring and long. Took me about a year of trying to watch those episodes until I just skipped to the end.And the ending was awful also. 
Boys over Flowers
Heirs
Kill Me Heal Me
You from the Star