There are many doctrinal differences between churches but at the very core of it, what defines a cult is whether that particular church believes the sufficiency of the work of Christ on Cross as his/her soteriological basis.
Ravi Zacharias says it well (from minute 4.00 onwards in the video):
"A cult is generally defined as that which claims to be rooted in historic Christianity but has deviated or abandoned the finished work of Christ or compromised on His Person. In strict Christian’s terms, Christ is not sufficient… It is important that we understand that Jesus said we are complete in Him, and we add or subtract, you can give whatever name we want, you are impugning the completed work of Christ on the Cross."At the conclusion of his talk at minute 7.00, Ravi added:
"To use the term “cult” in a general sense, in a general community, to a general audience, is not a wise way to do it. You use that in a setting of theological debates and dialogues and discussions. When the word is tossed around like that in a public setting, because of all the issues we have with people like Jim Jones and others, it brings baggage with the term that makes it much more than what a mere theological discussion would be. So that’s a term we leave for the classroom, not for a public arena because it says much more than what I think one is intending to say with a statement like that. We need to be wise and be mindful of the implication of a loosely used word. It is a very technical word reserved for a theological discussion around the table or a lecture hall."I agree with what Ravi said. The term "cult" tossed around, even if used correctly, carries a lot of negative connotations, or "baggage" as he called it; what more, if it is applied wrongly, it causes a lot of stresses, hurt, bitterness, shame and causes unnecessary divisions.