Everyone can identify Sir Paul McCartney's voice, or Tony Bennett's, or Frank Sinatra's. One simply must stop and take note when their voices are heard. Willie Nelson, Johnny Cash, Bruce Springsteen, Sting, and Taylor Swift also come to mind--there is that something about their voice and delivery that makes people stop and listen.
Fischer-Dieskau became, in his generation and genre, one of those voices, along with a bare handful of others such as Peter Pears. Like Frank and Tony, Dietrich understood that he sang in order to deliver poetry to the audience, and that the audience matters. The text always 'won', always shaped the performance, always guided the proceedings. He understood musical style, and how to sing stylishly. He understood the music he sang as entire pieces of music in which he participated, not as some sort of vehicle to carry his ambitions along. He knew entire scores, not just the bits he sang.
He also understood that the gent at the piano was his collaborator, not his accompanist. And he knew what to do in a recording studio, a great part of his genius.
Every time you hear a singer talk about his or her 'voice' as some separate entity, apart from the delivery of poetry in music, run the other direction. Otherwise, you will be treated to turgid performances in which everything, in technical terms, may be correct, but not worth the time spent listening. Go watch a baseball game, a soccer match, a golf tournament. At least those folks understand the goal of their efforts.
So much classical performing training I continue to encounter seems to be geared to simultaneously equipping students with great technical chops while suppressing their individual voices. It's happening now more than ever, in our age of programmed performing children. (To quote Rozanne Rozanna-danna, 'Don't get me started!') I was, to great extent, taught as a player that my job was to perform within the expected parameters, always give the conductor what he wants; and that most of the time, my opinion just did not matter enough to express. It was a shock when I began to find my own voice as a performer.
It is a blessing that as a young man, his teachers were not able (or didn't seek) to suppress this voice. The world owes them, and him, a great debt of gratitude.