To refine Pfilip
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
The same criteria apply to vocals
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.