Well, I think you need to be real with yourself. It's a lot about gut feeling, but there is craft to scriptwriting.
Ask yourself these questions:
What is the most compelling thing about this story? What about your story sparks your imagination? What are the good aspects of the script?
Does your story offer enough conflict? If there is any part of your story without conflict, get rid of it. We don't need people staring off, walking along, having mundane conversations about nothing. Do you have characters in agreement? Every character should be involved in some level of conflict with the protagonist. If this is not possible with the character you wrote, is the character really needed? This doesn't mean that every character has to be full on against the protagonist. Even if the conflict is small put it in there.
Does every page push the story along? Is every line needed? If it's not needed, it should at least be pushing a small conflict to develop your characters. If you have a part that is unneeded and doesn't house any conflict, there is no point for the part to exist.
Are you following the 3-act structure? Does your film give a clear singular point of view? For instance, the audience always identifies the first person to be introduced as the protagonist. They think, "oh this is who this film will be about. If you start out your film with a secondary character, your audience is going to think that secondary character is the protagonist. Like it or not, that's what happens. If your film does not start and end with the protagonist's story, you are in trouble.
Is there a clear goal throughout the entire film? After the catalyst (the point in the story where the goal is established, happens in 1st act) the audience should be asking themselves, "Will the protagonist meet the goal?" ("Will he get the girl? Will he blow the terrorists?").
Is your protagonist active or passive? Do things happen to him, or does he go out to pursue his goal? If your protagonist is passive, you are in trouble. For example, I had a script where a character starts experiencing the symptoms of a heartattack (audience asks, "Will he survive?"). Trouble is, this conflict is passive. A heart attack happens to you. So, instead, I made his goal to go and try to save himself by getting to the hospital. He is now going out and trying to accomplish his goal instead of letting the conflict passively happen to him.
Is there character arch? Does your protagonist change in how he tackles conflicts throughout the film? If he doesn't, the film will appear flat and meaningless. When a character comes across an obstacle where he has to change to get to the goal, it's compelling cinema.
Maybe you think there are some cliched scenes. If it is a comedy, what are the painful parts? If this is a drama, what could funny? There is pain lying underneath comedy, try to see the absurdities in a tragedy. Always think of the opposite of what is expected, and you might open new paths to uncliched ideas.
Is your script on-the-nose? I was watching star wars episode 3 the other day and the most on-the-nose line was uttered by anakin. In the film he sees the jedi as evil from his point of view. I can see George Lucas writing the script, "Gee how do I get across that from his point of view, the jedi are evil? OH I KNOW!" Obi-wan: But the Sith are evil! Anakin: From my point of view, the jedi are evil! Lazy, lazy, lazy scriptwriting right there. We discover character through their actions when dealing with conflict. We discover intent through the subtext of the dialogue. This is the way to develop stronger scripts.
Finally, is your story visual? Is there conflict that can be conveyed through visuals alone? For example, a battle scene is a very visual conflict; you see the action, you see the various obstacles and conflicts the characters are encountering. An internal conflict will never play good on screen because cinema is a visual medium so take advantage of it. If you have internal conflict, there must be an external conflict that represents such internal conflict in your characters. This is why adapting literature is so hard; literature can get inside a character's head and so forth, we can't with cinema. Unless you use a voice-over, and those are bad anyways. V.O.'s will risk you telling the action instead of showing and on-the-nose dialogue. I believe V.O's to be a lazy copout. SHOW DON'T TELL.
So think about these questions. Be real with yourself and don't be married to your work. Know what works, throw everything else out. A solidly structured script that can ignite the imagination is the most important piece to a film.
