Drama Rating I think, is very subjective to our personal experiences and mood at the time we watch the drama. There are no Objective way to rate a drama. We can choose categories, such as plot, chemistry between the actors, best picture, or best OST, but, when it comes to a drama we like, it's how it move our emotions. Did we laugh, cry, was surprise, bored or felt nothing at all, our emotions at the time we watched a drama dictates the rating.
For me personally, if I can watch a drama again and again and still love it, then I would give it a higher rating, than a drama that can only hold my interest for a short period of time. That said, Masters Sun, it's Okay that's love, boys over flower and heirs are dramas I can watch again and again, while healer and You who came from the stars, I can not finish it the first time around.
My two cents