So... here are my personal rules for good theater. Just as an aside, I have purposely not read any of the posts before me so that I don't end up accidentally copying someone else's thoughts. However, the catch is that I'll probably be repeating something someone's already said anyway. Apologies to all in advance.
1. Every plot point must be necessary. By necessary I mean that it must have ultimate relevance to the story, characters or theme in some way or another.
2. Spectacle must TELL. Everything on stage (or not on stage) should have a purpose and should communicate.
3. Lines should be tight and memorable (not necessarily all epigrams à la Oscar Wilde, however.)
4. Dialogue and especially diction should say something about the characters who are speaking. (Sometimes it's not what they say but how they say it.)
5. Time sequence should also have a point. For example, what is added to the plot by presenting the story in a chronological order? Backwards? In flashbacks?
6. Actors should disappear into the characters. One of my personal pet peeves when watching a play is when I'm too distracted by an actor when I want to know his or her character.
7. Whether it be embedded in the play's themes, the characters, or the plot, there should be an element of universality to which the audience can connect-- something that resonates with them and makes them identify with/care about what's happening on stage. Otherwise, one wonders at the end of a play, "So what?"
8. It must be memorable!!!!!
I realized as I wrote these rules how often I come back to the theme of necessity and purpose-- how nothing should be present without a reason. Sorry for the over-emphasis... this is probably due to the fact that I personally tend to err on the side of lots of unnecessary or pointless dialogue, characters, etc. and am trying to learn to pare down my work to the bare bones.