There was a period in the early 1990's when silent keyboards were in fashion. The consumer trying a keyboard in a computer shop got fascinated by the silent keyboard that reacted promptly to the light touch of a key. But the silent keyboard is not good for the experienced user. If, by accident, you press a key only halfway down then you don't know if your keystroke has been accepted or not. You have to look at the screen to check it before you press the next key. This interrupts the flow of keystrokes and distracts the brain.
A keyboard should therefore always have the click feeling. This means that the mechanical force against your finger suddenly drops down when the key goes down beyond a certain threshold. This is called tactile feedback, and it makes sure that you are never in doubt whether the key has been properly pressed or not. The tradeoff for such "click" feeling is some extra amount of time necessary to press the key. If you are after raw speed, you might need no tactile feedback, but you are more likely to make more mistakes when typing huge amounts of text.
The size and distance between the keys should of course fit the fingers. The keys should not be too hard to press, but, more important, not too light either. It should be possible to rest the fingers on the keys without pressing them down. Typists will get strain injuries if they cannot rest their fingers on the keyboard.
As there are certain physiological differences and variations of hand sizes for different people, the optimal distance between keys may vary from person to person. In ideal case it would make sense to make a few models with different parameters, much like clothes sizes, but in reality in would incur increased price for such solution. Hence you can usually purchase only "average" solution, which is not granted to meet your personal need, but it has an advantage of being cheap.
The keys on a keyboard should be organized logically into functional groups. Seldom used keys may be made smaller and put away in a corner or behind a lid.
Spaces between the groups of keys make it possible to distinguish the keys by just feeling with the fingers. This is an advantage not only to blind people, but also to trained users and touch typists who tend to look at the keyboard as little as possible. Most keyboards have tiny raised bumps or ridges on the keys F, J and 5 to make it easier to feel where the keys are without looking.
The QWERTY placement of the keys on an alphabetic keyboard is not optimal but was originally designed like this for mechanical reasons - the mechanic typewriters could not handle almost simultaneous presses and would "jam" - so the layout was designed with mechanical typewriters problems and specifics in mind, but for convenience and traditional reasons had been transferred to computer keyboards as well. Other designs are better when you are going to use computer keyboard, but the QWERTY keyboard has become the standard. The same is true for the placement of the numeric keys on computers and pocket calculators, with 789 in the upper rows. Other devices have 123 in the upper row, as shown in the page on standardization.
See also the page: The design of a button should reflect its importance.