I get what you're saying, but I'm not too sure what you want to discuss here. I'll just say that because most composers were also gifted pianists, many of their compositions were written in part for the piano first and then orchestrated. I definitely agree that piano music can be just as effective as string quartets, lieder or symphonies, that has a lot to do with the fact that it's much easier to write true polyphonic music for a keyboard instrument than for say a cello.
I'll profit of the opportunity to post one of my favorite disc:
Stravinsky - Petrushka / Prokofiev - Sonata for Piano No. 7 / Webern - Variations for Piano, Op. 27 / Boulez - Sonata for Piano No. 2 (Pollini / DG / 1972, 1978, V0)http://www.megaupload.com/?d=GX4XFSOS
A lot more modern than most of you are used to I think, but if anything, this is an excellent introduction to XXth century music. From the dazzling virtuosity and breadth of the Stravinsky, through the cold, brooding, mechanical universe of Prokofiev, to Webern's expressive austerity and to the final Boulez sonata, a self-avowed purely intellectual work that somehow manages to sound musical enough (to my ears at least, if you don't like it just skip it, I'd hate to see people missing out on the first three works because of it).
And anyone that doesn't believe in Liszt's composing abilities should give his sonata in B minor a try. I couldn't describe just how much I like this work, Argerich's recording of the work is excellent.