"What are the differences?"
Translation:¿Cuáles son las diferencias?
In this case then, it seems ambigious which of the two to use. We aren't providing the the person being asked with a limited selection of answers so why cual? Also if it is that the selection of answers be the requirement for cual why would asking what someones favourite film is be que? Theres only a limited selection of films available for a person to choose from, albeit a very large selection.
The reasoning I've seen previously is that "Qué" would imply that you want a definition of differences. So the answer to the question would be, "differences are what things do not have in common." Where as "Cuál" or "Cuáles" is asking what specifically those differences are.
Of course each question word can be used for other purposes as well. "Cómo?" Is frequently used as a single word to ask someone "What did you say?"