Obviously, there is the commercial definition: one that pays for its own development in a reasonable amount of time and costs less to operate than it takes in as revenue. I’ll ignore WoW here, but many of the older commercial worlds are commercial successes; even if they are not blockbusters. The trend in AAA worlds is that the development costs have skyrocketed so high that they are generally doomed to failure in the economic sense.
But what about hobbyists’ worlds? What defines a success there? When is a world successful?