- How hard you press the keys (Velocity Sensitivity)
- How you vary the pressure on the keys while they're held down (After touch Sensitivity)
- How quickly you release the keys (Release Velocity)
- How far you turn the modulation/pitch bend stick/wheel (Pitch Bend/Modulation Wheel)
- How long you hold a pedal (Sustain Pedal)
Those are the only means of affecting the sound during a performance other than by turning dials in realtime to affect sound parameters as you play. What I am really talking about are means of expression that allow you to ad different forms of expression that are available on an acoustic instrument like a guitar or a saxophone.
Side To Side Expression: Does It Exist On Any Midi Controller Keyboards?
Another new one if it doesn't exist would be to have an option on a midi controller to use side to side expression, that is by shaking the key to one side or the other would act as an expression channel like pitch bend or filter amplitude (make it assignable of course). There's enough distance between the keys on keyboards that it would be feasible and I don't think it would impact the lifespan of a keyboard's design.