Dropped these when continuing with them would have been a total waste of time ...

(1)  Pinocchio
(2)  Warm and Cozy
(3)  My Love Eun Dong

Just three among many.
Mine:

BOF
Surplus Princess
Switch Girl + Chinese Paladin 2

Among those I've watched of course !
1. One Fine Day
2. Loveholic
3. Exhibition of Fireworks
Completed: Marry him if you dare, Iris 2, Doctor Stranger,...they all have unintelligible plots. Special mention to the weirdness of Nail Shop Paris, just...no words. Also Blood should be mentioned. Kristen Steward is more expressive than the whole drama. 

Dropped: 7th Grade Civil Servant (I understood NOTHING), Secret Investigation Record. But the ones I will really NOT recommend are Secret Love Affair (the topic was interesting, but the execution was not - dense, boring, and at the same time superficial,...this drama seemed elegant in a way but they wasted the chance to really dive deep and the characters seemed to be written stupidly at the end) and The Golden Era of the Daughter in Law. I know this kind of family-DIL dramas are angst everywhere, but in this particular one the MIL just abused the DIL, even hitting her. The abuse was too excessive, couldn't stand it, but the nauseating thing was the moral side of the story, something like: this is the hard destiny of a DIL, withstand it and shine later! I literally couldn't see no more. Goddess of Marriage should be here too, just so bad, in the same way of justifying abuse, pain, disrespect...in general I really dislike this type of dramas for this reason. 
Love Rain
Dream High 2
Big
Fashion King => God I hated this. Actors had no chemistry at all and oh well in that drama it seemed like they had no talent to act. But they proofed me wrong in later dramas, but oh god, it was so bad.
Shut Up: Flower Boy Band : its too bad I cant handle watching it and I really like school dramas. this was the worst one ever
History of the Salaryman ,Rooftop Prince these are too silly dramas I hated them

1. Passionate Love (after 40 episodes of too many stupid choices, contrived obstacles and WTF moments, I just had to stop)
2. Tree of Heaven/Stairway to Heaven (because is same, is same so they're tied) (somebody please come up with a new plot for forbidden love. PLEASE.)
3. Ai Ore! Love Me! (I like Romcoms. I like Gender Benders. I like manga adaptations. But I didn't like this. It just seemed...dumb.)

(ok #3 is a movie but it made my list anyway)
I hate I hear your heart.

NVM. It's "Can I hear your heart". lol I didn't even bother to learn the title well. There are so many good ratings that I thought maybe if I finished it, then it would be okay. But it's not. At all. 
akai ito - How am I supposed to feel bad to lovers that never showed any sign they like each other lol. It's just a big mess
playful kiss - I don't mind cold male leads but there's a line where it gets annoying. Also the girl's actions were cringeworthy 
cinderella's sister - I'm still trying to finish it. I think I started watching it more than half a year ago. The first episodes were awesome and I loved it but after the fastforward in time it gets sooooo boring.
1.Temptation
2.Temptation
3.Temptation
1. Tatta Hitotsu no Koi
2. The Heirs
3. You're Beautiful
Out of the ones I've seen all the way through:
 
1. Faith - I watched this because so many people LOVED it. Plus, you know, Lee Min Ho (who I enjoyed in City Hunter). But I quickly became completely baffled by any praise this drama gets. I just don't get it. It's boring, repetitive, no character development, and sooooooooo much longer than it needs to be. I will NEVER understand the hype this drama gets. 

2. Orange Marmalade - I love supernatural-esque dramas. But this drama was so, so bad! The high school moments were pure torture; I think it could have been better if it was entirely set in the Joseon Era, but.....it wasn't. And everything that could go wrong with it did: annoying high school cliches, bad acting, over-used tropes of conflict. It just didn't work. 

3. Boys over Flowers - No explanation needed. Everyone and their mother has seen this (unfortunately). :/
1. boys over flowers 2. you're beautiful 3. boys over flowers
1. Nobunaga No Chef - The drama was plain BORING.
2. Ouran High School Host Club - What.. even... is... this?
3. Baking king, Kim Tak Goo. - Usually I can finish dramas even though it might be boring but for this one I just couldn't... By the last episodes nothing special happened, only the same thing over and over again.