Aman Ahuja:

>My curiousity is aroused, however.  What pieces _are_ usually considered
>the most "difficult" piano music? What does one mean by "difficult" in the
>first place?

Not a piano player myself, I can only tell you what I've heard:

Liszt: Transcendental Etudes
Sorabji: Opus Clavicembalisticum
Ravel: Gaspard de la nuit
Balakirev: Islamey
Rachmaninov: Piano Concerto #3

I suspect there are probably other even more difficult pieces, by composers
who don't get heard or played.

As to what constitutes a difficult piece, I would suggest the following
criteria:

1.  Finger agility: lots of notes very fast.
2.  Variety of touch, particularly at soft dynamics
3.  Architecturally abstruse works, difficult to make musical sense of.
4.  High degree of counterpoint: making the individual lines clear and in a
pleasing textural relation to one another.

Steve Schwartz