She must have been something special, both as a singer and person. They are calling her "a diva that wasn't" and many mention their profound experiences in her concerts. Here's one tribute to her.
I've really been astonished at the response on the blogs that I follow. This person captures it well:
Anyone who ever experienced Hunt Lieberson's magic knows what all the fuss was about. Whether she was singing Mahler's great paean to life and sorrow, Das Lied von der Erde, or some of his fragilely beautiful art songs, the world itself seemed to pause and listen. When she sang Lieberson's Five Rilke Songs at the Santa Fe Chamber Music Festival in 2001, it was as if pearl had been given sound -- a voice, as I wrote then, with a 'rich, violalike lower register and clear upper voice,' one that 'practically spanned a planetary system' in some of the far-reaching melodic lines. And when she embodied a character onstage, as in Ashoka's Dream, it was hard to imagine anyone else ever doing it as well.