As long as they're completely optional...

- Gestures might not make sense in many locations (tiny cubes with limited space, for instance), or prove problematic for Deaf individuals (learning, effectively, another signed language). That said, I totally want to select text, make a yanking motion with my hands, and have it get put onto the clipboard. - Eye tracking would be an immediate no-go in any sort of closed facility - Voice input is also a no-go in closed areas, and would be extremely annoying in cube farms - far more distracting than they already are.