I saw an article on the BBC website about this show over the weekend. It's interesting, to say the least.
As for what makes something a "cult," usually society labels groups of people who follow beliefs that society views as abnormal as a cult. Typically cults draw members from people who are vulnerable somehow, and attempt to take them completely out of their home life (ie away from any friends and family), and enforce sometimes extreme forms of control over them. Take Heaven's Gate, for example. The leader convinced every single member to dress a certain way, eat certain foods, and even drink kool-aid laced with poison, all through manipulative means. The same goes for "People's Temple" from the 70's where the leader managed to have all of his members move to a remote area in a foreign country, build him a town, and all live exactly as he told them they could. He also commanded women to sleep with him, told them who they could or had to sleep with, and also had all of them commit suicide in the end. Cults aren't just about sheer numbers, but more about brainwashing, control over members by the "leader," and the separation of the members from their original lives and everything in them.
In these terms, Scientology could very well be labeled "cult-like" or even just plain called a "cult."
