Actors
Genre
Synopsis
The feels it would give me
Score/Reviews
Actor/Actress
Plot

Generally I make sure to check to see if a drama has a high score or not. As more often than not, a high score will mean that it's a decent drama for one reason or another. Of course some times you get fanboys and fangirls who give the drama a 10 just because their favorite person is in there. Then I go to the next criteria and decide whether I know people in it and whether I like them or not. Lastly if everything fits the first two, I'll watch it if I like the plot.
Lol, I choose based on the synopsis and sometimes the popularity.

EDIT : And genre based on my mood.
Plot/Genres almost always
Actors/cast sometimes 
If it's completed I'll see how popular it was ^.^
1.Plot 
2.Actors
3.Length
Depends on how I am feeling.