As a pianist I can not reiterate the importance of having a piano do what it is capable of if it can not, then I am fighting a losing battle. There are simple guidelines that we can follow if we just take the trouble to learn a little bit of the mechanics of this wonderful machine which has remained the same for over 100 years.
If the keys feel mushy when pressing the key down, it might mean that the keyframe has not been properly bedded. This is a critical point to check because it can give false readings with other regulation problems if this problem isn't addressed first.
There are some measurements which may vary from instrument to instrument so I will give general specifications the technicians use. Key dip is the distance the key moves down. If it is deeper than 3/8 of an inch or 10.5 mm from the top of the ivory to the deepest point on most grand pianos it is too much. The distance of the ebony key when pressed down should be about the size of a nickel over the adjacent ivory key. Hammer blow distance is the space between the hammers at rest and the underside of the string. This generally is 47mm or 1 7/8 inches. You can make a little gauge out of a coat hanger to test your piano's blow distance. Then just have to technician adjust it or learn how to do it yourself. Key height is generally 2 ½ inches from the base of the key frame to the underside of the keytop. This is longer on larger grands.
These specs are general which the technician should know, they can vary from the size of the instrument, but not by much, for example small spinet pianos have a hammer blow distance of less than 1 7/8 inches and this dip should be just under 3/8 of an inch.
Too often we notice that the action his very heavy, it can feel like a concert grand even though it may be a fetal baby grand. This is an indication that something is wrong. Generally speaking, the smaller the piano the lighter the touch. Older pianos also have a lighter touch, requiring less pressure to produce a sound. This is because the parts i.e. hammers, shanks ,flanges and wippens are lighter than modern ones. Technicians call this gauge of touch, down weight. Touchweight analysis is a new buzzword that piano technicians call the mapping of piano action responsiveness. One very prominent jazz and classical pianist, Keith Jarrett had to stop his career due to chronic fatigue syndrome. His piano was too hard to play so he stopped recording his playing for several years. It was only after his technician altered his pianos action geometry, reduced hammer weight and removed friction that his piano was more comfortable for him to play. Now he has a more responsive piano and his improvisation has matured in a way due to the forced simplicity caused by physical limitations.
Action center friction can cause more resistance and increase the down weight. Older Steinway pianos used to get a green rust in their action centers, it is called Verdigris. Verdigris causes too much resistance and the only permanent solution to this problem is to have the action centers reamed slightly to remove the verdigris and have the center pin replaced. Most technicians opt to replace the parts with new ones rather than to go through the work of rebuilding the failing parts.
Grand Hammer flanges may be stiff, they should freely swing about seven times on their own. Tight hammer flanges causes resistance and repetition problems. If you have the technician remove the stack from the keys and gently twist it in a vertical position about 45°, you can watch the hammers follow the moving stack, and the sluggish ones can easily be identified. Hammers should be aligned to all three strings when the hammer is touching the string on the underside. On some grand pianos, when the una corda (left) pedal is pressed, only two strings are hit by the hammer. This causes less volume, but if the hammer moves to far with the una corda pedal, two notes can be heard because the neighbor string is being hit. Either the hammer flange will have to be moved with a hammer flange spacer, or the flange screw will have to be loosened to be adjusted and aligned under the three strings. If it is consitant that the adjacent note is being heard when using the una corda pedal in several notes and if the hammer is properly aligned under the three strings, then an adjustment can be made in the right cheek block. This prevents the left pedal from moving the string to the adjacent one. When repeating the notes sometimes a hammer can hang and the note will not sound. A cause of this may be that the jack is escaping too soon and not returning to the spot under the knuckle from where it should be at rest. By moving the jack, so that it is more under the wippen this problem can be eradicated. Lost motion is the empty space the key moves down before any of the hammers move. The key should not have more down more than 1/16 inch play before the jack moves. After 1/2 of the hammer blow distance is covered the dampers should be engaged. Having the dampers lift too soon causes more resistance and more touchweight. When the drop screw meets the repetition lever and the jack tender meets the let off button it causes a click. The click can be too pronounced if hammers are too low.