(So after thinking about what makes good plays, musicals, dance pieces, and performance art in general, I came up with this.)
Good theatre includes:
1) A story that can be comprehended and reflected upon by the time the lights fall.
2) At least one character that is easily likable.
3) At least one character that the audience can feel unsympathetic toward.
4) Some sort of conflict or journey... a change that takes place throughout the piece (this is not to say that a piece cannot end the same way that it started... something just needs to happen in between).
5) Visual moments that vary in range.
6) Sound design that vary in range (from silence, to vocal inflections, to music, or percussive elements).
7) Actors that will invest and commit to their character within the story.
8) A universal theme or quality that everyone can relate to in some fashion.
I'll probably be adding more to this, but these were the most pressing, it seemed.