for the way i rate dramas, the overall quality of the drama itself and my subjective thoughts, how much i enjoyed it, are important. i give 10 very rarely because the drama / movie has to be perfect in all its aspects + i have to be thoroughly invested and addicted it. there can be dramas i find quite perfect, but if the story does nothing for me emotionally, it will get not more than 9.5 from me.
here is a detailed explanation of how i rate:
10.0: perfect in every single way, made me feel a lot of things and didn't ever make me bored.
9.5: perfect, but there was a tiny something missing.
9.0: great! however, there was a thing or two that could have been better or more interesting.
8.5: super enjoyable! but something didn't 'catch' me, something was missing.
8.0: very very good, but there were things i did not like or the drama just wasn't able to actually fully catch my attention, make me addicted.
7.5: pretty good! probably really liked it at some point, but it was far from perfect and it lacked something important.
7.0: it was okay! there were things and episodes i really enjoyed but eventually watching it wasn't that exciting to me anymore.
6.5: potentially good but not really for me. probably got bored during it.
6.0: okay at first, then it got boring or something ruined it.
5.5: would have been pretty good, if something hadn't ruined it. if i finished it, i probably regret not-dropping it.
5.0: i don't doubt there are people that still would enjoy this, but very much not for me, probably never would've missed out on anything if i didn't watch it.
4.5: boring. something that i'm sure there are much better versions of.
4.0: hard to watch, things tend to seem too random or don't make sense.
1.0-3.5: bad. very much not recommended. don't start this if you can and spend your time better.
when choosing a rating for a completed / dropped drama, i strictly go by this system. so if you look at my completed list, all the dramas with the same rating have pretty much the same value in my eyes.
p.s. if i drop or skip through the drama, i do not allow myself to rate the drama with higher than 6.5. :D