I honestly don't know how I should rate the dramas I've seen, I change it 24/7 because I'm never completely sure :') Could you guys share your rating system, I might use it ^^

I try to rate my completed dramas without being biased about it~ I try to think, "In the world that the drama is created around, was everything that happened enough? was it good enough? could there have been more? could things have changed? could the ending have had been better?" and then I rate my completed dramas based on that. If I find myself really disliking a drama because of something or some actor's acting~ I drop it. Like this~ the dramas that I do finish are pretty alright by themselves. (Mainly because I drop anything and everything that can not keep my interest.)

Thus, my rating system is like this:

10: Perfect. Nothing more could have been added~ the very drama was good in the way it was built from the beginning.
9-9.5: There was something wrong with the ending or the chemistry between the leads. But awesome otherwise.
8-8.5: The second female leads got their way a lot too many times/ a few bad cliches. Good enough though~ I still smiled through it all. Well most of it.
7-7.5: I'm on the edge~ I like it, but it could have been so much better. Disappointed, but it's alright.
6-6.5: How did I survive through this? I really don't like this, but I'm too nice to give it a lower rating because I liked a character or an OTP.
5-5.5: This was unbearable. Usually, these dramas are present in my Dropped list.
0-4.5: This was the worst thing to have ever been created/ Wow this actor/actress really ruined everything with their ridiculous acting. What a waste of time and effort. NEXT!

I rate hard (I've never given a 10 because then I have no where to go if something better comes out) and I try to use all the numbers.  In my mind (and on my list) there should be way more dramas scoring between 3 and 7.5 and very few at either end. 

So to me something with a score of 5 is average.  It's probably still better than most of the dross on western TV, it probably still has a good story, if you like the lead actors you'll probably like it.  Here's my scale:  
0.5 - 2.0   =   F   =   You'd probably have more fun repeatedly punching yourself in the face.
2.5 - 3.5   =   D   =   D for Don't Bother unless one of the main actors is your Bias 
4.0 - 5.0   =   C-  =   Average, give it a go - you might like it 
5.5 - 6.5   =   C   =   Sound, solidly put together, worth watching
7.0 - 8.0   =   B   =   Very Good, add it to your PTW list without question.
8.5 - 9.5   =   A   =   Excellent, make this the next thing you watch. 
10           =   A+ =   Perfect  Also, I'm not going to claim any kind of impartiality with scoring.  

I try to be objective, sure, but sometimes I just love or hate a drama with only emotional justification.  Here are two examples:

Faith:  I know the direction was bad, I know the cinematography wasn't brilliant, I know writer Song had trouble getting them to stick to her story but the depiction of a solid no nonsense love (even with all the nonsense around it) makes it my most rewatched drama and it gets a 9.5. 

Descendants of the Sun: Had it ended at episode 14, I probably would have given it a 7.  I felt so ridiculed by the writers and producers for following them along for the last 2 episodes that I threw that 7 out the window and it's very bloody lucky to get the 2 points I begrudgingly awarded.
Honestly, I'm not the kind to give bad grades, but here's (approximatively) how I rate movies and dramas:
10- either it was perfect, either I really liked it. Yes it's different: sometimes I see that it's not a perfect movie/drama, but the feels are here (hi Yamaneko Tantei~~~~) 
8.5-9.5 it was nearly perfect and I liked it a lot. Not giving 10 because there was one thing I didn't like (chemistry, ending) or just because I didn't feel like giving it a perfect score. It's very good nonetheless and it's a must-watch 
8- very good. Like above, there was one thing I didn't like but it's not that bad either. It's a must watch if you like the same kind 
7.5- it's the last "good grade" for me. It means I liked it but not that much; either the quality of filming/actors wasn't good, or I really didn't like the story. 
7- same as above but worse
6.5: I really didn't like it but there still was one or two things I liked. I really don't recommend watching these unless you're a pure fan of an actor. It was probably a boring watch
6- the worst grade for me. I usually don't grade below 6 because I still respect the works hats done, but 6 means it was very badly done and/or boring. 
5.5- well I've never given that grade but it'd probably mean it was very bad; maybe one element was good. 
0-5: don't watch it~~~ 
I start rating after the 5th episode. My rating can go up or down the rest of the way until the end. 

10- Perfect, or I loved it despite any flaws.
9- Awesome but had some minor flaws that I couldn't ignore.
8- Good. I liked it.
7- Average. Nothing special. 
6- Eh...below average.
5- Pretty bad or I am torn on whether I feel it was bad or good. 
4- Why did I not drop at this point?
3- This is so bad, I wanted to see just how bad it got.
2- WTF?!
1- I can't even.... (I don't recall ever rating this low)
0- Did not make it long enough to rate before dropping/n/a
10: Excellent, exceeded all expectations with little room for improvement. I don't recall having given this high a rating yet though.
9: Very good, despite very few flaws and/or parts where I felt could be improved. Would definitely recommend, left a good and lasting impression on me.
8: Good, pretty solid all around with perhaps a few minor flaws/felt slow in some parts or a slightly disappointing ending. Would recommend regardless.
7: Not bad. Still enjoyable but too many flaws to ignore, or lacked in a fair amount/felt like it dragged in some parts/unsatisfying ending. 
6: Below my expectations, just about okay. Average at best.
5: Not great, barely watchable.
0-4: Don't bother with this, it's a waste of time / don't plan on watching / dropped / got bored.
My rating is quite subjective because it's a rating on how I enjoyed something... Nothing objective on it. But objectively, if a drama is good, it means actors are good or good enough for a perfect story for example. If everything is objectively bad you probably won't like it. So I wouldn't care too much about your rating system... Just give any score first and you'll find your own way.

Anyway, mine would be : 
10 : unforgettable, a real masterpiece. I will remember this drama for a long time
9 : almost perfect, there are some flaws but hidden by something very special
8 : really good
7 : enjoyable but I wouldn't recommend it
6 : not bad, certainly not good either
Below : why did I watch this ?
But I don't often use the scores between 0 and 4 because I usually drop the drama before
10 Masterpiece, either perfect or emotionally so satisfying that I didn't care about minor flaws. 
9.5 Excellent, almost perfect but not a 10 because something bothered me so much that I couldn't ignore it.
9 Awesome, it has some minor flaws but it left a deep impression on me.
8-8.5 Good. It was very enjoyable but I was disappointed because of some parts.
7.5 Near good, Enjoyable despite its flaws but I didn't like it that much.
7 Decent, it was okay and still enjoyable but not memorable.
6-6.5 Above average, it was watchable but I only liked a few things about it.
5-5.5 Average, it wasn't bad but nothing special or too cliched. 
4-4.5 Below average, basically bad but there was something I liked about it.
3-3.5 Bad, don't waste your time on this. I was utterly disappointed.
2-2.5 Terrible, I didn't like it at all.
1-1.5 Crap, WTF?
Pretty much 8-10 everything
Mainly 9, as every show I seem watch seems perfect when it probably definitely isn't
10, if it completely made me lost into the show
For me it depends, I don't rate solely on my preferences, so for example you might see a number of high rated dramas/movies in my dropped list:

So if I completed something :

1-5-Varies stages of disliking it and it was generally not a good story in my opinion, but I needed to know what would happen in the end.
6-7.5 I hugely enjoyed some aspects of it, but as a whole I think it was just ok. I might even re-watch the bits that I enjoyed.
8-9 Yup it was a solid story and I personally found a lot that I liked in it. Would re-watch, especially if there was somebody else who'd watch it with me.
10-  I LOVED IT and it was generally a very good drama. No doubt I'd watch it again and again and again and ummm yet again!

If I dropped something:

1-5 Varies stages of dislike, but basically "Oh Dear God, what the heck did I just watch?!!" I didn't like it and the story made no sense/ bad acting(my opinion).
6-10- Varies stages of ok I appreciate it was well done, but it's just not my cup of tea or I accidentally spoiled myself and what I found out just isn't to my liking.
10: my favorites
9: amazing, loved them
8: great, enjoyed them very much
7: good, enjoyed them

If I'm debating whether to give drama 8 or 7 I usually add in count if I wanted to re-watch it.
Also everything 7 and above are something I could recommend to people

6: average. like this is the rate I give when I don't know what to give. like the drama might have been good but there might also be something boring or annoying.
5: ok, just below average. these usually are boring and after finishing them I might think why did I watch this.
4 and below: boredom and horribleness increase exponentially the lower the number gets lol
10 - impossible
9.5 - masterpiece
10 - series that took my heart and time!!!
09 - the series that i enjoyed watching 
08 - good series 
07 - the missed some elements to make it good
06 - i hate them!!!
So interesting to see how many different ways of thinking there are! For some people 6 is a pretty good rating and for others 6 means it's pretty bad. Well, that is good to keep in mind when you look at a drama's rating because even though you can see how many people rated a drama this or that number, there is no way of telling what people mean by it. 

For me, my rating system has changed a bit because I've watched more and more dramas. In the beginning I gave a lot of 8-10 and 7 was mediocre. Then I felt that this made the scale too narrow; I found that my "good" ratings needed a broader range. (And if I really dislike a drama I drop it and in that case I don't rate it.) I feel that it makes sense to make 5 a middle rating for the dramas that are half good/half bad. (Of course it's not those dramas that make me very happy to be a drama addict; it's the 8 and above that really makes it worth it, but even a mediocre drama can have something good about it that makes me not regret watching it.) My ratings measure how much I enjoyed the drama and what makes a drama enjoyable is very subjective. Sometimes I can understand, intellectually, that a drama is very good but I still don't connect with it all the way emotionally and that's what matters most. Rating a drama is not the same as grading student papers; this is for enjoyment! So my rating changes sometimes but for now it's something like this:

10: I've sold my heart and soul to this drama!!
9: amazing
8: very good
7: good
6: okay, but with flaws/was boring at times/too many things I dislike; still more good than bad
5: it was watchable but also annoying; half good and half bad. 
4: I disliked most of it but it still had something that made me not drop it
3: it was pretty bad, I very nearly dropped it - so it's the lowest rating on my list

2 and lower would mean even more stages of dislike and I don't find it meaningful to keep watching if I dislike it that much, that's why my ratings don't go that far.