I think Acting = Plot.. If one is mediocre or really really bad, nobody would rate it 10/10 or even 8/10.
Good acting with bad plot = meh. still can stand it a bit
Good plot with bad acting = what a waste.
But I'll go with acting. I don't really mind if a plot isn't that good given that the actors/actresses did a good job, but the acting... If the acting done in a drama is really bad, I probably won't finish it.