For me it's pretty easy to rate a Drama...
1st - Story. Is it original, common, boring, ridiculous
2nd - Acting. Do the actors fit, how is the acting in general
These are actually my two factors for rating. I've never really gave a Drama more points because of an actor, that I especially like - I'm being honest here.
For example: Personally I don't like
Toda Erika, but I love
Liar Game, because she fit the character and her acting was in my eyes pretty good. On the other hand, I really like
Yonekura Ryoko, but I gave (for example)
Doctor X a 6, because it was common and had not many twists.
I only give a 10, when the Drama blew me away, if all the cast did a splendid acting and if the story was new, fresh and unique.
I hardly give less than 3 points, but of course there are cases like that... when the Drama is REALLY ridiculous, the acting horrible or what's mostly the case: a really bad original-story-destroying-Live Action.