Well, I dived into Asian drama because I was learning Japanese. And we all know, once you do, you never come back...... :)
Regarding how I rate dramas:
1) I rate by GENRE. Even the attempt to compare a cliché comedy romance with melodrama or action-based drama seems pointless to me. We are talking apples and pears here. You can say that you prefer one or another, but there is no objective scale on which you can apply a comparative rating.
2) I look at what each drama ASPIRES to be. So a cliché comedy romance can get 9 points from me, if it hits all the right marks, while a seemingly "great" action-thriller can get less, because it failed at what it tried to do - give a logical plot-setting etc. You want to be an outstanding, for example criminal, drama with a really clever plot? Well, if you fail at that, I will rate it less than a shallow romance that did all what it promised to do, even if that was only fluffy and well-known romantic scenes with evil chaebol moms and the typical jealous rich bitch.
3) LOGIC. That's the gold aspect when it comes to serious dramas. You want to sell a clever plot? Then it got to be logical. IN EVERY FREAKING EPISODE. I am the person who finds all the plot-holes or fake plot-twists or illogical developments. It is like math, really. I do not care how pretty the faces of the actors or how awesome the action-scenes. That is the difference between a fanfiction with big budget and a masterpiece - it has no holes. (Dramas which are mostly based on emotional development should deliver some logic in that aspect, too. But since they usually do not attempt to give super-intelligent plot developments, my expectations are not as high here.)
4) GOOD DIRECTION. Most dramas do okay in this aspect, some are even ingenious, but there are a few out there which actually failed because of the poor directing and/or cinematography. Cuts and camera positions have to be as carefully arranged as a poem. If you're into that stuff, you know what I mean.
5) NOT FAN-GIRLING. Gosh. You can throw hearts at the screen all you want, but although there are actors/actresses I really like or even think beautiful, I couldn't care less how they look when it comes to the quality of the drama.