From NPR:
What, exactly, made him so great? A research team in Europe wanted to answer that question, so it looked into the science behind his voice. Professor Christian Herbst was part of that team, which just released its study on Mercury; as a singing teacher and a biophysicist, Herbst says he was intrigued by Mercury's technique. According to his research, the key lies in Mercury's vibrato, which differs slightly from those of other classically trained singers.
“Usually, you can sing a straight tone, but opera singers try to modulate the fundamental frequencies,” he says. “So they make the tone, if you like, a bit more vibrant. Typically, an opera singer's vibrato has this frequency of about 5.5-6 Hz. Freddie Mercury's is higher, and it's also more irregular, and that kind of creates a very typical vocal fingerprint.”
You might be able to hear that vocal fingerprint in the vocals-only version of Queen's hit song “We Are The Champions” below.
More here.