In my opinion, "good theatre" is in the eye of the beholder. What I might find entertaining in a certain performance may not be what others enjoy. With this being said I firmly believe that TRULY good theatre is a performance that can successfully satisfy not just my interests, but the entire audiences. Personally, I believe there are many aspects of theatre that all work cohesively to make good theatre. For example, I find good theatre has...
Humor/Comedy. Who doesnt love a good laugh? Personally if a play can make me laugh its already pretty amazing. Whether its witty dialog or good acting, a good laugh could never hurt.
Romance. Whether its a happy ending or not, Im a sucker for a good romance. Romance for me usually gives stories their magic. Historically, romance has been the basis for many famous writings (Shakespeare is a very obvious one)
Drama. Where theres romance theres drama. I have never seen romance in theatre without some type of dramatic complication. I enjoy theatre that has conflict. Nothing in real life goes strictly according to plan so when I see plays without conflict I find I get bored very quickly.
Suspense/Mystery. Where theres drama there is usually a mysterious character or secret that is sometimes known to the audience and sometimes not. This is one that can either make good theatre or not. I dont want to be able to figure out the end of the play at the beginning, I like having to keep guessing.