To partly answer your question, I think virtuosity is to some extent related to the aural impact (even, sometimes, the visual impact) that the piece makes on an audience. I think there are pieces which are obviously difficult, pieces in which the difficulties are not readily apparent to an audience, and pieces which are not really that hard, but which can be made to sound virtuosic. Off the top of my head, an example: if the Chopin G min Prelude is played particularly agitato, it will sound pretty virtuosic, despite not being especially difficult.
Virtuosity has more to do with how well you play than what you can play.