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To refine Pfilip's categorization: Within music we can distinguish several qualities:
  • expressiveness - music can deeply appeal to the senses
  • adequacy - music often serves a specific purpose: to help create an atmosphere or mood, to help telling a story, or as part of a celebration or ritual (including concerts)
  • aesthetics - music tends to generate its own standards of aesthetic value, which vary from genre to genre; some artists manage to set new standards on their own
  • technical prowess - the display of musical skill can be impressive and educational
  • originality
The same criteria apply to vocals and lyrics.

For example, Bob Dylan is a singer of limited technical skill - although his timing is immaculate - but his voice is very expressive and adequate for his lyrics. Therefore it would be short-sighted to regard him as a poor singer.