Stevie had used the Moog extensively during his golden period and was very familiar with the capabilities of synthesizers of the day. I believe his complaint was essentially boiled down to: while they provided new sonic vistas, they did a terrible job of reproducing the sounds of real instruments. Samples were poor and lacked dynamics.

What he wanted from Kurzweil was a synth to mimic an actual piano. He wanted to play a 'piano' on stage without the heft and burden of an actual piano. Further down in that wiki:

"The Kurzweil K250 is generally recognized as the first electronic instrument to faithfully reproduce the sounds of an acoustic grand piano."