In all seriousness, it's a good question. If you asked me that question, I still struggle with defining what JWs actually do believe. When you ask a JW you're more likely than not to get a list of things that they don't believe, usually a list of what they think that "Christendom" believes.
I suppose that the defining belief relates to the above cartoon. They absolutely believe what will happen to them if they don't believe what they're told.