I think what "distinguishes" bad scripts is inauthenticity. But that requires some defining.

An authentic script / story acknowledges that there is the world of the writer and the world of the audience, and these two worlds are different. Authentic writers know that they will have to perform some difficult work to take their gut instincts and turn them into a form that will appeal to the audiences' gut instincts (in my view, that "difficult work" is what writers learn from learning the craft). An inauthentic story is just the writer talking to him/herself. Inauthenticity comes through quickly in action descriptions, dialogue, and other features that seem just to be technique but which reveal the willingness of the writer to sacrifice self in order to connect with the other. You don't have to know the full plot / storyline to detect inauthenticity.