For me, good theatre must generate a strong emotional reaction. Good theatre has to make me laugh, cry, long, love, hate, envy, respect, value, etc. It has to make me FEEL. It should also be in-your-face, rule-bending, line-crossing fun. Good theatre should be hawt. :)
Good theatre should inspire me to change my life. It can be a drastic change, or a change so small it seems insignificant. It should make me want to be a better person, or take voice lessons, or dye my hair. Or maybe it'll inspire me to write a play of my own.
One of the most important theatrical elements, I think, that makes theatre good is characterization. If you think about it, our personalities include facets of characters we have seen in plays and movies. Even characters we have read in literature. Every time we exit the theater, we take a little piece of the performance with us. This influence has the power to affect our (major) life decisions. Sometimes I wonder, is this really me? Or am I emulating a character from a play I read/saw? I think good theatre should challenge your sense of self. It should help you discover who you really are by bringing to the surface your untapped goodness.