The thing that's really great about all our acoustic musical instruments is that when you push on them, they push back. I don't mean this simply in the normal force sense. Consider a guitar. You've got a string which you press over a fret with your left hand. This can be naively emulated by a switch. But with the guitar, the timber and pitch change depending on how you fret the note, on the finger pressure and position and motion. With your other hand, you might be plucking the string in any of a number of different places, with your finger or with a pick. The sound is affected by your attack angle, how hard you pick, the pick's composition, and so on.
A guitar string is clearly a complicated system. There are lots of variables at play. But more importantly, it's a coherent system. It makes sense to us a physical object that can be manipulated. When you pluck the string, you can feel it vibrate in your fretting hand. When you bend the string its tension increases. If you amplify it, you get the sense that you are physically touching the sound.
(This is incidentally, why audio latency absolutely KILLS when doing amp simulation)
The experience playing a wind instrument is similar. While a saxophone may appear to be something you blow into that has keys, things are really far more complicated than that.
Keyboard-based instruments are a little different. Unlike most any symphonic instrument, the piano actually has relatively few parameters per key. There's note velocity... and that's about it. The various sustain pedals also apply. The piano's design trades single note expressivity for the ability to play ten of them at once.
It should be noted that computer synthesis (procedural or sampled) of keyboard-based is very convincing. They same cannot be said for any other instrument.
Now, what about new kinds of control systems? Most of them tend to fall into two categories. One tries to improve on the piano harmonically, by coming up with a better arrangement of where the notes go. Here's an overview of some: http://sequence15.blogspot.jp/2010/03/alternative-keyboards..... They try to fix the fact that it's hard to play in different keys on a piano. Whereas on the guitar you can learn a single scale or chord and move it up and down the neck to transpose, things change radically on a piano keyboard.
The second category is those like the Seaboard, which try to add new dimensions of control to a regular piano keyboard. Another example is the Contiuum (http://www.hakenaudio.com/Continuum/) It's very common now to have both velocity and continuous pressure sensitivity (aftertouch) on a regular keyboard as well as various side controllers for dealing with pitch or an abstract "modulation" parameter.
These controllers nearly always buy into the separation of control from synthesis. It makes perfect technical sense. But most of the instruments we would consider to be "expressive" don't work that way! In fact piano-style instruments are pretty much the only ones that do.
Which leads my to my point: A control mechanism should be considered together with the instrument it controls. It's fantastic that this new keyboard has all these new dimensions that you can map to sound, but what is it REALLY good for? What is the instrument that wants to be controlled in this way? The spiffy new control surfaces nearly always leave this problem unsolved and thus remain little more than novelty items.