Greatness
I guess 'greatness' is defined by the beholder. A great voice, just like a great guitar player, is more than mechanics, range, tone, etc. It matters not if Tony Bennet or Elvis, or Sting, or Sinatra are considered superior vocalists, if they don't hit you where you live. I can 'appreciate' Pavarotti ability all day long, but I can't relate to his musical genre. Jim Morrison touches and massages a deep, dark place than no one else even acknowledges(with the exception of David Gilmour), so I suppose 'greatness' is an overrated concept. What matters most is what reaches you and your response to it.