In the era of digital pianos, there's a common grievance about how digital piano's key action feel is nowhere like acoustic pianos. The reason for this is simple. On a acoustic (“real”) piano, the key action mechanism is hammer based. That is, there are a series of intricate mechanical apparatus, such that when a key is pressed, a seasaw (fulcrum) is pushed to cause a hammer to jack up and hit the string, which vibrates and makes the sound. If the key is pressed hard (quick), it eventually cause the hammer to hit the string strongly. Similarly, if the key is tapped lightly, the hammer hits the string mildly, causing sound of lesser volume. However, if the key is pressed too slowly, there's not enough force to jack up the hammer. Thus, one has to hit the key with some minimal velocity to make a sound.
Also, because the whole physical mechanism involves elaborate fulcrums, levers, springs, dampers, hammers etc, there's this intricate tactical sensation when playing on a acoustic piano. Namely, the resistance from pressing the keys is nonlinear with respect to the velocity level. If pressed too softly or slowly, no sound are made as there are not sufficient force to engage the hammer. If pressed very hard, one can feel the momentum of the hammer, where a loud sound will be made without having to press the key all the way down. By resting a finger on a key and play it like a child plays seasaw, one can feel the varying weight of the hammer on the other side.
In contrast, modern digital piano's keyboard has very different feel to them. Because the musical instrument is electronic, there's no string to hit, and no need for elaborate hammer or leveler mechanism. All that is required to register the sound is a device that measures the velocity of the key being pressed. Thus, digital piano's key mechanism is often such that the there is a linear relationship between velocity and volume: The harder you hit the key, the louder it sounds, and that's it. The resistance of the key is also uniform, unlike that of acoustic piano. As if there is a soft marshmallow underneath the key, where regardless how you press the key or whether it is already halfway depressed, its resistance is the same. Scientifically even, or artistically inert, depending on your point of view.
One easy way to realize the performance effects of these two actions is trying to play scale in the softest manner possible. (ppp pianissimo) On a digital piano, this is easy to do. On a acoustic piano, it takes more skill to perform, because one needs to apply just the right pressure to get the hammer flying but at the minimum speed. If the fingers are not well trained, then on some notes there'll be no sound, while others will be noticeably loud.
I love the feel of acoustic piano actions. It just feels great, where when passing certain point the key suddenly lost much of its resistance because of momentum, and this depends on how hard you are pressing it.
I'm also what one might call a “scientifically oriented” person. Here we have two keyboard actions, one with linear velocity/volume relation, while the other curved. One must note that on acoustic pianos, the curved velocity/volume relation is not by choice. It is by physical necessity that their key action have a curve velocity/volume relation. Keys need to engage hammers, hammers need to hit strings, and it is not feasible to make the key velocity, the hammer velocity, and string vibration amplitude all correspond in a linear way. Whereas in modern age, linear velocity/volume relation keyboard action is everywhere with digital pianos.
Examples of digital keyboard with the even resistance as described above can be seen in most weighted keyboard sythns or so-called “digital pianos”. Yamaha s08, Roland fp-3 are two examples.
With modern technology as of 2002, it is possible to make a acoustic piano with digital piano action. To make a acoustic piano with linear velocity/volume key action, one replaces the traditional hammer/fulcrum set with a velocity sensor and a computer-controlled hammering device. In this setup, the hammer will hit the string in accords to the key's velocity sensor. This way, the piano will have a scientifically even key-action yet produces sound by traditional way of vibrating strings.
On the other hand, one can make a digital piano with acoustic key-action. That is, take a traditionally-built acoustic piano, then replace the strings with a electronic drum. So, now the hammer will hit the drum, and the drum is a velocity measuring device, which feeds the velocity into sound-generating electronics that makes corresponding loudness and pitch. This way, we have a digital piano with acoustic piano key-action.
I think both of these ideas has been tried, but the resulting product being expensive and uncommon. For example, the Kawai MP9000 digital piano has somewhat acoustic key action design, albeit much reduced in the key-action complexity. The Kawai MP9000 and its sequel MP9500 are renowned for their imitation of acoustics's key-action feel.
On the other hand, as far as i know the Yamaha Disklavier and GrandTouch digital pianos are build like acoustic with genuine hammer key mechanism, but produce music through either dummy string with microphone pickup or as MIDI output.
Although the acoustic piano key feels great, but because of its non-linear velocity/volume relation and out-of-necessity nature, i find such property logically and technically inferior. If one considers the parts and building process and weight and durability and economic factors, then it is more obvious that the physical key action of acoustic pianos has little competing edge to the digital pianos's key-action, apart from tradition.
For this reason, i recently am selling my MP9000 and got myself a Yamaha s08 synth, to satisfy a psychological need of being a avant-garde tech-head.