A bad quality might be a 'sin' or 'flaw', and a good quality a 'grace' or 'virtue'. Is there a term like this for a neutral quality? If there is no term is there an idiom or slang?
I would note that the word quality is, itself, neutral:
quality: an inherent feature : PROPERTY (Websters)
Other, similarly neutral, words that come to mind include:
attribute: a quality, character, or characteristic ascribed to someone or something. (Websters)
characteristic: a distinguishing trait, quality, or property (Websters)
trait: an inherited characteristic (Websters)
feature: a prominent part or characteristic (Websters)