|December 29th, 2011|
The problem with the piano or keyboard is that you've traded off a lot of expressiveness for capability. A pianist can choose how hard to hit a key, which changes how the string sounds when it is struck, but that's really it.  A violinist, however, can control the vibration of the string continuously, being much closer to directly manipulating it.
This is a tradeoff we see all over the place: a concertina is easier and faster to play than a harmonica, but it has limited control over the sound. The penny whistle and flute, harpsicord and the harp, bombarde and bagpipe; they all follow this pattern. People didn't stop playing the violin after the invention of keyed fiddles, and not just because they're complex and expensive: the farther you get from directly manipulating the sound, the more control you give up and the more your sound is limited. 
Seeing this relationship eveywhere, I decided to make some charts. They show, for a kind of instrument, what possibilities there are as you make different tradeoffs on complexity, cost, automation, and capablity. Consider the free reeds:
I made a few more:
The full set, including others I didn't draw:
no holes end blown flute pan pipes pipe organ kaval no holes fipple flute slide whistle recorder, penny whistle pipe organ fife, wood flute concert flute shawm crumhorn bagpipe oboe jaw harp harmonica melodica concertina, accordion reed organ didgeridoo bugle trumpet, tuba trombone bell xylophone toy piano water glass glass harp glass harmonica hornpipe chalumeau clarinet saxophone berimbau harp zither autoharp harpsicord psaltery mountain dulcimer hammered dulcimer piano diddley bow violin, viola, cello, bass guitar, mandolin, ukelele, bass guitar viola de amor nyckelharpa hurdey gurdey washtub bass erhu
Update 2013-11-19: The viola organista is another instrument that belongs on this chart. It's a bowed string instrument, where pitch is chosen by selecting from among appropriately tuned strings. It should probably go under the bowed psaltery.
 This is reflected in MIDI: a note is defined as a note (0-127) and a velocity (0-127), followed by, some time later, an 'end of note' command. (There are also aftertouch, channel pressure, and pitch bend messages, but your standard midi keyboard doesn't generate them.)
 This sounds negative, but I don't mean it that way. The tradeoffs can be good ones: I like that my mandolin has frets because it makes chords much more practical to play in tune. I use a pick because I can play much faster. I like my penny whistle because I'ven't learned how to shape my mouth to play flute.