Characteristics of a Cult
Primary characteristics of a cult are:
A Single Powerful and/or Wise Human Leader
I don't believe that Camping or any of his followers profess him to be powerful, but I think most believe he is wise and do listen to him to learn the meanings of scripture.
The cult leaders word, or teachings of the cult become absolute truth over shadowing the Word of God
While Camping says over and over "the bible says", in reality he is teaching "what the bible says according to Camping". Despite the many proofs and even absolute absurdity (in some cases) of the things he teaches, his followers believe and profess what he teaches.
Separates Itself From Society
Harold Camping clearly teaches that true Christians are to depart out from the church and fellowship only with like-minded believers.
Emotional Control or Coersion
The emotional ploy is of utmost importance: the very destination of one's soul. Either you believe the Bible the way Camping teaches it, or you will never go to heaven.
No Meaningful Accountability
By relegating the "visible church" to the rule of Satan and with it casting out all Elders, Deacons, Pastors or any form of church/spiritual authority (except God, of course) followers are left only with their own conscience.
Comparison to Jehovah's Witnesses
Charles Taze Russell and the Bible Students
Jehovah's Witnesses grew out of the Millerite movement of the 1840's. In 1870, Charles Taze Russell and others formed an independent group to study the Bible. In 1877 Russell jointly edited a religious journal, Herald of the Morning, with Nelson H. Barbour. In July 1879, after separating from Barbour, Russell began publishing the magazine Zion's Watch Tower and Herald of Christ's Presence, highlighting his interpretations of biblical chronology, with particular attention to his belief that the world was in "the last days". In 1881, Zion's Watch Tower Tract Society was formed in Pittsburgh, Pennsylvania, to disseminate tracts, papers, doctrinal treatises and bibles; three years later, on December 15, 1884, Russell became the president of the Society when it was legally incorporated in Pennsylvania.
Watch Tower supporters gathered as autonomous congregations to study the Bible and Russell's writings. Russell firmly rejected as "wholly unnecessary" the concept of a formal organization for his followers, and declared that his group had no record of its members' names, no creeds, and no sectarian name. The group became known as "Bible Students".
Since its inception, the Watch Tower Society has taught that humans are now living in the last days of the present world order.
In the years leading up to 1914, 1925 and 1975, the Society's publications expressed strong expectations of Armageddon or the establishment of Christ's kingdom over the earth occurring in those years. The Watch Tower Society now teaches that Christ's personal presence was revealed in a spiritual Second Coming in 1914 and it is impossible to know precisely when Armageddon will occur, but that it is imminent.
Harold Camping and Family Radio
Similar to the Jehovah's Witnesses: Harold Camping is preoccupied with end times, separated himself from the church, started his own means of distributing information, predicted a number of dates for end time events and later changed them to "spiritual events" rather than nullify his prior teachings.
The real key is separating himself from the church. Any sect that rejects the Christian church as a whole (not simply errant congregations or denominations) treads on dangerous ground of becoming a cult. It is perhaps too early to determine where Harold Camping / Family Radio will fall.