The term "good theatre" brings to mind the image of a "bad theatre" being banished to the corner for misbehavior. I believe that catharsis is found in good theatre, for both the performers and the audience, and that each performance of each run of a play is always different in its incarnation and moves everyone in a way specific to them.
I'm tempted to make a list of everything I like in a theatrical production, but that would just be my preferences and I don't know if that is reason enough to make something officially "good". That being said...
I love theatre that embodies the idea of Gesamtkunstwerk, which integrates theatre, music, and visual mediums into a complete work. I love theatre that engages all of my senses.
I love reading a play or watching a performance and feeling transported to another world. Most often this happens when there is a wonderful female character that I imagine myself playing.
I applaud plays that have bold messages, but I don't believe that every piece of theatre needs to be completely earth shattering to all in attendance, shaking them to their core. Take, for instance, Spamalot. Not a very deep play, but I'll be damned if Tim Curry and Sara Ramirez singing Monty Python tunes doesn't make good theatre. It's too good.
Maybe good theatre is what you need it to be at the time, so that a pitch perfect, awe inspiring performance for one person is, at the same time, less than moving for the one sitting next to them. But does that make the piece any better or worse? Who gets to dictate that?