My main interest has always been learning about the ways in which quantum mechanical systems are different from classical ones. Of course, at the most basic level everything is quantum mechanical; however, many physical systems can be described to an excellent approximation by the equations and concepts of classical physics. Finding out when it is all right to do so, and when one should use, instead, the (typically) much more computationally demanding and often counterintuitive formalism of quantum mechanics is not always an easy task
