I'll volunteer the theremin for its total lack of any visual reference points, infinitely adjustable 'scale length' and the fact that you have to hold your entire body totally still to maintain a constant pitch!
Definitely, but I have heard somewhere that it is easy to learn but hard to master. The hard thing is that hardly any teaches it and playing is freeform, often players experiment and come up with their own technique.
Definitely the hardest instrument to play:
- No feedback (apart from sound) when hitting note
- Volume and pitch controlled indepedently of each other by opposite hands
- No defined note positions
- Hard to play one note after another without it sounding "blurred".
- Have to keep very still with note
The Continuum also sounds hard as it has no clearly defined notes, basically you can play inbetween the conventional notes, Jordan Rudess from Dreath Theater plays one aswell as Pedal Steels and Keyboards.
...and I could also imagine playing a guitar minus the frets as hard...
...has any one heard of the gusle? Probably not so hard as it is only one string but interesting sound, almost haunting and middle-eastern in nature but basically any musical instrument that you need to learn by hand positions (violin, theremin, gusle) is hard enough just to get the note you want.