Having settled into my position as the Library's first screenwriter-in-residence, I've begun reading the 44--a fantastic and daunting number--scripts that have been submitted. And I've noticed a common problem. It is a common problem because it's so fundamental and specific to film and TV writing.
Screenwriting needs a strong story.
Remember that a screenplay can only describe what the audience sees and what they hear. That's it. It's a rather crude medium.
The novelist can describe a character's feelings for pages--for the length of the novel even. But the screenwriter has no such luxury. If it can't be photographed or recorded, it can't be in the script. Consequently, a screenwriter can't discover their story as they write--novelists often claim to have this drive them through their writing. Screenwriters MUST have a clear idea of what their story is--whose story it is, what their goal is, who or what the obstacle to achieving that goal is and what is at stake. And the stakes had better be close to life and death, even in comedy.
That kind of structural rigour can mean that the "quality" of the writing isn't in the beauty or poetry of the sentences. It's in what the sentences ultimately describe. It's the story that lies beneath the actual writing in the screenplay. That's why writers who aren't particularly deft stylists can still be great screenwriters because their stories and their characters leap off the page--not only by how they write but by what they write.
This is a fundamental. Screenwriters must determine WHAT before they determine HOW. What happens? Rhett Butler rejects Scarlet O'Hara. That's an example of what. That's the story's basic component. This is WHAT happens.
Notice that knowing what happens does not reveal in any way HOW Rhett rejects Scarlet. (A lesser writer may decide that HOW Rhett does this is to say, "Frankly, hon, I just don't see this relationship going anywhere." A better writer actually decided that HOW Rhett does this is to say, "Frankly my dear, I don't give a damn." But in either case, the WHAT is consistent.)
Without knowing WHAT your story is doing, no amount of fancy writing matters. "How" still matters, but only after you figure out the "what".