Here are excerpts from two articles at Script.com: 12 Signs of a Promising Script and What Readers Look For to give a script a recommend. This is just a checklist, go to the links for the full articles and explanations.
Meet the Reader: 12 Signs of a Promising Spec Script
1. The script is short – between 90 and 110 pages
2. The front cover is free of WGA registration numbers and fake production company names
3. The first page contains a lot of white space
4. I know who the protagonist is by page 5
5. The premise is clearly established by page 10
6. Something interesting/entertaining happens in the first five pages
7. The first ten pages contain plenty of action
8. I can tell what’s going on
9. The dialogue is short and to the point
10. The script doesn’t begin with a flashback
11. There are no camera directions, shot descriptions, and editing instructions
12. There are no coffins: Amateur writers love to adorn their scripts with lots of irrelevant bells and whistles – fake posters ... illustrated covers, graphic novel adaptations, mix tapes containing the songs featured in the scripts, ... specially produced promotional merchandise – key chains, postcards, bobble heads, etc. ... most of the scripts that accompany this junk are usually just awful ...
Meet the Reader: What I Look For
(A year later, Ray Morton wrote another piece. A longer, more detailed article. Possibly he felt writers weren't listening. I'm including more from the list which has 18 points. Still - go read the whole article.)
1. A reasonable page count. As soon as I open a script, I immediately check to see how long it is. If it’s over 120 pages, then the script already has a black mark against it.
2. Something interesting in the first few pages. A good script begins with a scene or an event or an incident that is exciting or funny or intriguing or scary or spectacular
3. A clear premise. The premise is the core concept of your story (a shark attacks a New England beach town; a young couple fall in love on the maiden voyage of the Titanic;
4. An interesting protagonist. The protagonist is the script’s main character – a character with a strong goal whose actions in pursuit of that goal generates the story’s main events and drives the narrative forward from the beginning to the end.
5. A worthy antagonist. ... . The antagonist must be formidable enough to provide sufficient opposition to foil the protagonist for the length of the script. If the antagonist is a person, then I want him to be a colorful, interesting character, but not so colorful and interesting that he overshadows the protagonist.
7. A story that develops the premise. A good script is one in which the story springs from the premise and spins that concept out through three acts to a logical and exciting conclusion. ... nothing is more satisfying than a story based on a great premise that makes the most of it.
8. A story that brings a fresh twist to its genre.
9. A plot I can follow. We’re in the era of non-linear storytelling – screenwriters today seem determined to avoid presenting a plot in anything resembling a straight line and so go out of their way to tell tales in out-of-order fragments, flashbacks, asides, dream sequences, ... If you want to employ these gimmicks ... don’t convolute it to the point where I get lost in your narrative machinations, because if I can’t understand your story, then it’s a good bet I’m not going to recommend it.
10. A plot with momentum. A successful dramatic narrative is one that builds continuously from the inciting incident to the inevitable climax.
11. An exciting climax that resolves the story’s central conflict. A lot of specs contain climaxes that are action-packed, but that don’t resolve the narrative.
12. An unpredictable ending that is also logical and satisfying. ... when going for an unpredictable finale, writers often come up with denouements that come out of nowhere, that are not a logical outgrowth of the events that lead up to it. ... Unpredictable and twist endings only work if they also make sense.
13. Dialogue that is clever, characteristic, and brief. For me, good dialogue is dialogue that makes its point with wit and charm, that reflects the character of the speaker, and that doesn’t go on and on and on.
14. Tightly-written action. As anyone who has read more than a few scripts will tell you, action is the hardest stuff to read. ... I (and most readers) prefer it when the action is written in brief, easy to digest paragraphs of only one, two, or three sentences and that can communicate the essence of the sequence without describing every thrust and parry, lock and load.
15. A script that can be a movie. A script that can be a movie must also have a scope and scale that can be produced for a realistic budget. It must also not contain material that is inappropriate for the target audience (in other words, if your script is intended for a wide, mainstream audience, it cannot contain overly graphic sex and violence, something many young screenwriters fond of exploding brains and exposed, engaged body parts often forget).
16. An original voice. By this I don’t mean snarky or smartass stage directions – those are tiresome. What I mean is a point of view or a sense of humor or a way of looking at characters, scenes, and the world that are fresh, original, and unexpected.
17. A script that does what it is supposed to do. In other words, if the script is a comedy, then I want it to be really, really funny ... If it’s a horror movie, then I want it to scare the beejesus out of me ....
18. A feeling. A script stands its best chance of getting a thumbs-up from me if I experience a strong emotion at the finish – if I’m feeling very happy or intensely sad or amazingly inspired, or incredibly relieved. The best movies provide us with a moving, transformative experience. The best scripts do too.
