“Gestures lack critical clues deemed essential for successful human-computer interaction. Because gestures are ephemeral, they do not leave behind any record of their path, which means that if one makes a gesture and either gets no response or the wrong response, there is little information available to help understand why. The requisite feedback is lacking. Moreover, a pure gestural system makes it difficult to discover the set of possibilities and the precise dynamics of execution. These problems can be overcome, of course, but only by adding conventional interface elements, such as menus, help systems, traces, tutorials, undo operations, and other forms of feedback and guides.” [...]
“Gestural systems are no different from any other form of interaction. They need to follow the basic rules of interaction design, which means well-defined modes of expression, a clear conceptual model of the way they interact with the system, their consequences, and means of navigating unintended consequences. As a result, means of providing feedback, explicit hints as to possible actions, and guides for how they are to be conducted are required.”