I’m not looking for anything at all in a new piano work that I wouldn’t also be looking for in a new string quartet that I might come across, or a new chamber work for mixed instrumentation, or an orchestral piece, or a piece of music for any combination of sound sources that asks to be listened to repeatedly, thought about, lived with. What that “thing” is, is of course impossible to define accurately in words. But for me it does have something to do with possessing an idea or conveying a set of ideas that expands the sum total of my life experience, with speaking equally to my conscious and subconscious selves, and all the while being true to its own convictions and inner logic. If the work is successful to my way of thinking, then the fact that it is written for piano is simply a bonus, because then it means I can share my ideas about that piece of music directly with listeners.
I really don’t care at all about a work for piano being “well-written” for the instrument, or “idiomatic,” whatever that means. Thank goodness Beethoven cared less and less about that later in his life or we wouldn’t have pieces like Op. 101. When pianists speak about wanting composers to write idiomatically for their instrument, what they often mean is they want to be able to show off the particular sets of physical skills they have developed in learning to play Liszt, Ravel, or Prokofieff—in other words, what their parents paid good money for them to learn in music school. Or they want to draw on the same emotional “cues” with which they are familiar—they want to be able to make the same contorted facial expressions they make while playing Brahms. If the composer doesn’t write a piece that hits the audience over the head with the performer’s obvious “virtuosity,” then it’s often judged a failure, a poorly-written work. I guess that’s been true since Clementi‘s time. But most pieces in which technical display is the central focus bore me eventually. Hence the genius of the great piano etudes in history, from Chopin to Ligeti, in which the particular technical challenges manage to become absorbed within conceptual issues explored in each piece.
So my admittedly simplistic-sounding answer to the original question would have to be: if I think it’s a great piece of music, how the piano is being used does not matter in and of itself. The idea is the only thing that really matters to me. If you’re a composer wanting to write me a piano piece and I tell you that not only do I love playing Gaspard de la Nuit but also that Cage‘s “Cheap Imitation” is one of my favorite works for piano, that’s not going to give you much to go on, is it? That is, if you are looking to write a piano piece from the externals inward, rather than from the core out. And personally, I’m a “core-out” kind of guy, and the music I tend to be drawn to is usually of that sort.