What makes the 1984 society so successful?

This all depends on what you consider as successful. Oceania is "successful" at having a population that is brainwashed out of intense fear and conditioning. The outer party, like Winston, are drones of the state. They are a mass of existence simply there to support party ideology. There is no debate, decent or even creativity. The Party has no opposition, not even in thought. The Proles are simply too ignorant to care what happens. As long as their primal needs are met, they ask no questions. In this sense, society is "successful" in perpetuating its own existence.