Kudos to futurebird on an excellent description of a playwright. I reiterate: A playwright is a crafstman/woman. As such, there are many considerations that such an artist must take into consideration when crafting a play.

Most good plays (although there are some exceptions include the following attributes:

  1. MDQ - the Major Dramatic Question. The question of major importance in the play; the issue that keeps the audience members glued to their seats until it's resolved. Take Tennessee Williams' The Glass Menagerie as an example. The MDQ there is: Will Amanda find a gentleman caller for her daughter?
  2. Hook - comes at the very beginning of the play. Gets the audience involved and interested - in effect, keeps them from walking out of the theatre. Can be anything from an exciting opening moment to an odd bit of dialogue that leaves the audience guessing. In The Glass Menagerie, Tom's opening monologue - it tantalizes but doesn't give away too much information.
  3. Strong dialogue - varied, believable dialogue between characters. More on this later.

The plot of a play is generally structured as follows:

  1. Exposition - The very beginning of the play takes place in the "everyday world" of the characters. It lets the audience know what day-to-day life is like in the world of this play, gives character insights, and gives the audience information crucial to understanding the play.
  2. Inciting incident - Something out-of-the-ordinary that happens to get the action moving. Without the inciting incident, there would be no play. Introduces MDQ. In Menagerie, when Amanda finds out that Laura hasn't been going to typing school.
  3. Complications - Just what it sounds like. Obstacles that keep the protagonist from attaining his or her goal. These make the play interesting and cause the action to build. This is a cliche, but it's useful: A good play is an emotional rollercoaster. One moment the playwright tricks the audience into thinking that everything will turn out right, that a happy ending is at hand; the next, in waltzes a complication and the rollercoaster is on its way down again. But the complications must constantly build, build, build. In Menagerie, some complications include Laura's shyness, Tom's desire to live his own life, the husband who walked out on Amanda, the friction between Tom and Amanda, Laura's disability...and more.
  4. Climax - Point of no return. Also the point of highest emotion and action. Culmination of all conflicts; its resolution answers the MDQ. In Menagerie, Laura's kiss with Jim.
  5. Catharsis - Realization, due to some piece of information garnered during the climax, that allows the protagonist to reach a final resolution. Because of this, the protagonist will either become aware of a way to get what he or she wants, or lose it (often by rejecting it willingly). In Menagerie, when Amanda finds out that Jim is "going steady."
  6. Denouement - Falling action. Ties up loose ends. The shorter the better.

A few words about crafting effective dialogue:
There are three different levels of dialogue.

  • First level is simple and direct. It means exactly what it says. A character says "Shut the door" for the simple reason that she wants it to be shut.
  • Second level is outwardly much like first, but there is added subtext. Very useful in characterization; can also bring out the humor in many situations. A character says "Shut the door" to assert her power over another character, or because a past experience gave her a paranoia about open doors, or because she doesn't want anyone to overhear an upcoming conversation...you get the idea.
  • Third level uses metaphor or anecdote to get the point across. A character wants another character to shut the door, so she says, "You know, somebody else came into this room without closing the door a few months ago. His head is still on a stake out front."
You want to try for a variety of dialogue levels in your play, but the primary one should be second level. Characters should only directly reveal information (through first level dialogue) when they have a great deal of pressure put on them. That's pretty true to life; most people tend to avoid saying exactly what they mean. Also, too much third level dialogue can start to sound flowery and self-important. Second lends itself to subtlety and humor well, and sounds most realistic. But all are powerful when carefully employed.