I'm a big believer in asking the playwright questions about the play as a way of helping the playwright write the play he or she wants to write--not the play I want to write. Ask yourself these questions as you enter the revision process--answer them honestly--and help yourself to a better play.
Is the speaker's name ever on one page, while the dialogue that goes with it is on another page? (If so, get them together.)
Are the margins for dialogue and stage directions consistent? Are the speakers' names and the scene/act headings the only things centered?
Is my title page businesslike, without being overly flashy? Does it have the necessary contact information (name, address, phone number, email address) unless the submission guidelines tell me to do otherwise?
Have I eliminated ninety-nine percent of filler words like "well," "uh," "OK," "all right," etc.? While they are meant to make dialogue sound "realistic," they don't really add anything.
Have I punctuated the dialogue accurately? Have I gotten someone else to read it out loud in front of me so that I can hear if the punctuation makes sense? It's super important to put periods, commas, dashes, semicolons and whatever else you're using where they belong. It's the only real opportunity you have to communicate the rhythm of the lines to the actors.
Have I run a spell check? Have I proofread by reading aloud to make sure nothing has slipped through? Often, you can misspell a word into another correct word that your spell check won't detect. Have I given the play to someone else who has a good editor's eye?
Do I avoid dialogue which is only there to "tell" about the characters? Can I replace it with an action of some kind? For example, instead of a character telling us he is afraid of spiders, he could jump onto the sofa and scream for help.
If there are long monologues, do they have a good reason for being there?
Are the stage directions clear, concise and grammatical?
Do I use the stage directions to describe what happens but not to write a novel or long descriptions of characters' feelings?
Do I write the stage directions in the "active voice"?
Is it clear which character is supposed to do an action or perform a stage direction? Don't assume that it's obvious--usually, you should mention the character specifically.
Have I avoided line readings (e.g. "angrily") except in crucial cases?
Have I given a specific time and a specific place (e.g. a living room, not merely inside a house) at the beginning of the play?
Have I introduced each character with a one-line description (age, gender if it's not obvious, and a phrase of description)? This is crucial to help a potential director or producer determine who could be cast in the role, or simply to help a reader get a handle on your play.
Is each character distinct and well-developed? Is each character's speech consistent with his background and education? Do the characters sound different from each other?
What if you removed a character from the play? What would be lost? (Edit Villarreal, one of my professors, suggested this exercise, and it's a great way to make sure that you don't have two characters who basically fulfill the same function.)
Is each character's behavior and actions believable? Try "trapping" your characters, so that they feel they have no choice but to do what they do.
Does each character have a unique position in the play? In other words, if two characters fulfill pretty much the same function in a play, how can you make them different?
Are the relationships between the characters clearly established?
Do the characters change? Static characters aren't as interesting to play.
Did you pick the characters' names for a reason? As a sidenote, be careful of naming characters too similarly (e.g. James and Jack).
Other Content Issues
Does the play have a clear conflict with a beginning, a middle and an end? Does the conflict build as the play goes on? Remember that two characters arguing isn't conflict. Conflict is driven by characters trying to get what they want.
Are the stakes high enough? It has to be crucial to each character that she gets what she wants.
Is there a ticking clock? Time pressure always creates additional tension.
Is what happens in the play a result of choices the characters make, or do outside events dictate what happens? Strive for the former.
If the play requires research, do you have your facts straight?
Is the tone of the play consistent? You don't want a play to be a farce for the first ten pages and a family drama for the last ten.
Do you give the audience new information, or do you merely tell us things we already know? Audiences get bored without new information, and remember that while news may be new to a character, be careful if it's not new to us.
Every play is its own world that you create. Are the rules of that world consistent?
Is your play's title both catchy and fitting?
Does the play begin at the right point? Sometimes a play begins too early when it should begin in the middle of action.
Does every scene have conflict? Characters who desperately want things don't ever stop trying to get what they want.