At some point you watch something happen before your very eyes. Some dudes somewhere have a concept to what it is you're going to see and have produced it to be watched. It starts at some point. When it ends can be confusing.
If you like what you see, it's certainly good enough. I mean, you watched it and liked it, didn't you?
The longer you can talk about it, the better. This is why some really shitty stuff makes for good theatre. A good bitching is often more thought provoking than fluff anyway.
Perhaps the worst possible theatre would be something you can't think of anything to say about at all. This is why planks of wood make the worst critics - they probably won't say anything and just make you feel bad.
I am tempted to say that the worst possible theatre would make the whole audience vomit all over eachother and wallow about in it, but I can't be sure, though it sounds pretty funny.
Maybe the worst possible theatre would just kill the whole audience. Many would contend that mass homicide is not good theatre. By that logic, the opposite of that would be the best possible theatre: theatre that makes the whole audience live.
The vomit thing is still funny to me though.