I've been thinking about common frustrations with musical instruments and why I usually find strings (oppose to stringed) easier to play than keyboard instruments (or stringed instruments with frets). It pretty much comes down to scales. I think I remember music as a set of relations between an undefined base note and the present note, remembering them like a long array (or, for songs with multiple voices, an array that has other arrays branch off and rejoin as appropriate). That particular reference note is indefinite, and so for "perfectly natural instruments" like my voice, I can shift keys with only a moment's effort if I'm trying, and at other times not even notice. I like nonfretted stringed instruments (I can play string bass and violin) because if one is willing to use nonstandard fingering, it's an easy mapping between my mental representation and the string - provided I never use the open string, I can play entire songs a quarter note off (or really arbitrary amounts off) from the tuning (so long as the strings are tuned to themselves - have the fixed tonal relations they're supposed to by tradition to each other). Compare this to the keyboard - not only is one locked to whatever the piano is tuned to (let's ignore fancy pedalwork one can do on some pianos), the keyboard codes precise markers for the relations between notes. It also means that I can't rely on a secondary set of relations - that between the current note and the next note, to know how far to move my fingers (or if I need to move them "up" or "down") - without memorising a bunch of tables. I love the sound of the piano (and pipe organ, and accordion), but I don't really like the interface. Fretted guitars are kind of in-between by my book.
I think an ideal instrument would be a set of three strings about 4 feet across over some kind of wooden body, designed to lie on someone's lap as they sit down, with them plucking with one hand (or alternatively some kind of footpedaled plucking with some kind of clever string selector) and moving their fingers over the unmarked strings with the other. Ideally, the strings would be fairly distant from each other, notewise, to give the instrument a very wide range (the downside to this is that if one were to want to play chords, they would sound funny, and playing notes far enough from each other on the string length may be impossible without that foot pedal idea). Hmm. Maybe that's not such a practical instrument. Still, dreaming about new musical instruments is fun - it'd be cool to get together some people to actually build all the instruments anyone came up with that would be workable.