Hello everyone.

In order to develop the plot, the major ideas in the drama/movie there has to be that specific content to lead to watchers from point A to B.

DEFINITION OF FILLER
Fillers (not to be mixed with fillers in anime) are the things that are put between these A and B. Eg. from not knowing the person to marrying them, or, from being born to being the knight who slays the dragon. That what happens between the major plot events are what are fillers. E.g. in Naruto the main thing is the fighting, and the fillers are practicing and learning new skills.
Now on to the fillers in Korean drama/movies. Before a person A marries person B, there has to be the filler, that what makes the main plot, marriage, sensible and natural.

EXAMPLES OF FILLERS
For some reasons, these fillers are generally misunderstandings, fight over power in politics, hospitals,  quarreling, fight over heritage and scandals to name a few. Some make sense, no doubt. However, some of these are quite random e.g. a person is sent to hospital only to be okay in the next episode, babies are switched after birth and one or several cars appear from nowhere attacking people.
These fillers were okay if they were somehow introduced and/or hinted in advance. However, let us say in episode 6 we are introduced person C. In the episode 7 to 11 that person causes problems. The reason, which is revealed in episode 11 was family problems. Okay, I understand. That person had social problems, but how about introducing or even hinting there might be some problems? Additionally, for some reason, this family problem is fixed as fast as it appeared. Lastly, after the problem appears and before it is solved, the people have to misunderstand each other, quarrel, lose their control and do something reckless. 

However, maybe the most extreme type of fillers are those that took place before the drama actually happens. D-DAY SPOILERS: The main characters parents where in an accident, The father died and the mother was operated. The main character´s brother operated the mother, which resulted her being in eternal coma. This results in the bad brother relationship portrayed in the drama. Okay, sure, that is quite shocking, but how about preparing this filler in advance? How about somehow hinting of this in some kind of form? Next one, the one woman talks with a "bad, corrupted" man. They just discus, and oh, they are father and daughter. That is a cool filler, but how about preparing that filler? How about telling from the beginning that they are father and sister? D-DAY SPOILERS END

The problem here is that it is possible to throw in any type of back story to almost any type of situation. I some times create these kind of fillers myself, here is one:person B causes trouble to person A.  A:s father is the brother to the banker (who is person B father) who lent money to the sister of person A, and this sister then tried to committed suicide but failed. The sister became depressed and A:s family hid from A:s the fact the main character has a sister.

THE POSSIBLE PROBLEMS
The fillers are sometimes so random, appear from nowhere and so many, that they actually become the main plot in the drama/movie, although these fillers were originally meant to fill the gaps between the major events in the drama/movie. This leads to several facts:
1. The main plot (not the fillers) becomes slow and draggy
2. The drama/movie loses its original idea, since the main plot and all the random fillers blend together to a mess. The writers basically have too much stuff in the plot they cannot decide what they should do with all the stuff.

PERSONAL OPINION ABOUT THE FILLERS
Everyone hates each other, always scream and are super depressed and gloomy. Then for a moment everything is super great, everyone is fantastic only to allow the writers to throw some new problems that will occupy the next 5, 6 episodes.I watched the new drama "Puck" (if that is the English name) and thought in advance: this is about ice hockey. This drama must have some great games, and I cannot wait to see the secret training camps. But no, again do these fillers with difficult human relationships appear and it becomes so depressing. Were you not supposed to play ice hockey?
What is the idea of fight for political power in a hospital? Just treat the patients.  NODAME CANTABILE (KOREA/JAPAN) SPOILERS Why make the decision to disband S orchestra so political and difficult? In the Japanese anime and drama they hardly discussed that subject. I believe it just ceased to exist. NODAME KANTABILE SPOILERS END.

LAST COMMENTS
People like what they like and some obviously like these random fillers. I agree that they are shocking and entertaining in some aspect, but I am not fond of them. Since someone will say this, I say it in advance: Yes,it is wrong to generalise (at least if it is something negative) and there are always exceptions However, one has to generalise in some points to see the entirety.

Thank you in advance if not for commenting, then at least reading.
you made many good points :). i think that the problem of korean dramas being dragged out and/or losing their pace are because they are longer than japanese dramas. then when you compare two same dramas (as in nodame cantabile case) japanese dramas look more dynamic because they lack significant amount of fillers. and i honestly think that the duration of kdramas is the main problem even in original kdramas, not only adaptations. the episodic and oh-so-random character stories that cause some stir for 2 episodes is that writers are sometimes 'desperate' to fill the story. and same goes for 'politicalising'. the fight for power seems to be the to-go plot when writers seem like they dont know what to do. i've seen it in almost every drama and it doesnt matter if the dreama is about high-schoolers or doctors. it seems like it is the widely accepted cliche plot.. the thing i hate the most is when episode ends so dramatically and you are obviously being tricked into watching next episode so you watch it and the minute it starts they get rid of the dramatic element and make it just casual misunderstanging that gest solved in 2 sentences. and then im angry because wtf, i've been tricked into watching another episode?? whats even worse, many times almost whole duration of episode is pretty boring but again, in last few minutes some /drama/ happens, which baits you into watching next episode -_-

also, the lack of foreshadowing might be cause by the fact that script is written while the drama is being shot so when they come up with something breaking its is not easy to fit it in the previous story? idk that is just my guess :D
i honestly believe that korean dramas are getting better in past few months? lately i have noticed that some recent dramas have less than usual 16 or 20 episodes and mostly it is the planned lenght and not that the production was cut short. also there are more minidramas/webdramas. it opens 'new' posibilities for interesting dramas being made and also new ways of working with story fillers so im hoping korean dramas are on good way of getting good pace :D

These fillers were okay if they were somehow introduced and/or hinted in advance.

I wholeheartedly agree with this. I don't mind cliches, but I hate it when they introduce stuff like birth secret, amnesia or dark, hidden past in the middle of a drama simply to make it interesting without even hinting at these in early episodes.

And I think egg-mont is right. I suspect that Kdramas abound with fillers because of the live-shooting system. Even if the scriptwriters have a clear idea about their story (or had completed the script prior to shooting), the script can be rewritten anytime based on how the viewers (e.g. rating) respond to the story so far.
The events that occur between points A and B is known in fiction writing as the 'plot'. The story is the overarching tale that's being told.
Let's take a drama about Samurais fighting for freedom - that's the story, what the drama is about. The plot is the circumstances the protagonists find themselves in and the actions they take to get out of it - plot.

Fillers are normally non essential events added to a drama to prolong conflict and or to 'fill' the episode count. Skilled writers avoid fillers in their scripts. Not so skilled ones, like the Hong Sisters imo, flood their dramas with them.

At the risk of sounding a bit elitist I'd say most Korean dramas aren't made with thinking people in mind (it seems to me). Most of them do require you to turn of your brain and take a lot absurdities at face value. For some people -- even super intelligent people -- this is fine, but for me all the dumbed down nonsense gets to be a bit too much. Which is why these days I'm veeeeeery picky about the dramas I choose to invest my time in.

"I agree that they are shocking and entertaining in some aspect..."

I would say only if you're just starting out in dramaland and are not yet aware of just how repetitive 'these fillers', as you call them, are. They basically exist in every last drama you will ever watch ... and when you do find out, you'll either lose interest in dramas or become very picky about what you watch, like I have.