Aside form the Human voice...

Every voice and instrument has it's own unique expression. I have a fondness for a well recorded vibraphone,i.e Lionel Hampton style.

Strings that can be bent provide additional range for expression(try that on a tradional piano).

Rcp
Why do you say Cello?

Number of notes wasnt criteria. Previous post said guitar could do everything piano could. I listed one thing it couldn't do.

My vote goes to the kazoo. Spoons would be a close second.

lets offer up jimi hendrix as evidence. there are lots of great 'expressive pianists' but jeez louise, to this day what he did with an electric guitar is still looked upon as more than skill....how about magic?