To be a quality screenwriter, you must see the emotional and physical details of your human race.
Your ideas should always be grounded in truth (truth of the experience, truth of human race, etc.) Imagination then, is best utilized to move your ideas beyond acknowledged truth.
Truth plus imagination equals brilliant screenplays. One needs the other. Each alone will result in a screenplay that is either false or boring. (EG. Fallen Angel. A true idea inside a fictitious story involving a 12 year old girl and a pedophile.)
Audiences should lose themselves so much in the story and the characters that they forget their audience status. They are on that spaceship. In that room. Riding that horse. Kissing Mel Gibson.
The 4 most important elements in screenwriting are theme, story, characterization, and structure.
CHECKLIST FOR AN IDEA
The idea must promise conflict.
Conflict is the heart and soul of screenwriting.
In short, never put 2 people in the same scene who agree with each other.
The best scenes are "why are these 2 people arguing and why are they both right?"
Nobody wants to see the story about the Village of the Happy People.
2. Make sure your Idea can go the distance
The Beginning. The Situation. The Idea.
A good idea promises to reveal itself completely around page 17.
The Middle. The Complications. The Plot thickens.
Cause and Effect. Push and Shove.
Here, you must develop the dramatic complications.
From page 17 to approximately page 85.
The End. The Conclusion. The Climax. The Catharsis. The Wrap-up.
*Pick an Idea that promises lots of complications, and ways to spin an audience into different emotions.
*Make sure your idea can carry the whole 100 to 110 page script, a one-note story will not suffice.
3. Make sure your Idea can be visualized, dramatized, and verbalized.
Don't pick Ideas where most of the drama happens in the mind.
Artists, writers, actors, architects generally make for boring heroes because their inner conflicts are not easily visualized, dramatized, and verbalized.
Always pick stories that scream for visualization. Not mere "Talking Head" movies.
4. Who Cares?
"Who Cares?" is a test you should give to every idea
Make sure there is a significant audience who will care about your story, who will want to see it.