Deryk Barker replies to me:

>I'd certainly not want to omit Haydn, Scriabin, Rachmaninov, Busoni,
>Medtner, Rzewski, Grainger, Stockhausen, Messiaen....

Well, I started this tempest in a teapot.  However, I'll stand by my
criteria.  All of these composers wrote piano music.  Whether they tell
us something new about 1) piano writing and 2) deeply idiomatic about
their understanding of the instrument can be argued with.  Haydn's really
nothing that shows us anything different than a lot of his contemporaries
in their keyboard writing.  Scriabin derives from Chopin.  Rachmaninov,
Busoni, and Medtner seem to me to play around with Liszt and Chopin.
Stockhausen to me does interesting stuff, but not particularly idiomatic.
Grainger, Rzewski, and Messiaen come very close to making my list, and
I thought very hard about putting them in.  However, something held me
back -- probably timidity.  As for Jim Tobin's nominations of Satie and
Poulenc, I can't call Satie's music great music, but not something all
that new, and Poulenc's piano music, though interesting (even beautiful),
doesn't really seem to me his best work.  I've always Poulenc's best
keyboard music in the accompaniments to his songs.

Steve Schwartz