1)Harmonic accuracy.
Which in turn encompasses or is affected by many of the factors that have been mentioned (tonal balance, harmonic content, accuracy, clarity, resolution, coherence, lifting of veils, freedom from distortion, etc.). To me "harmonic accuracy" is the most significant determinant of how "real" the instruments sound. I realize that by lumping its contributing factors together I am begging the question :-)
2)Clean transient response.
3)Dynamic range.
4)Frequency extension.
5)Image scale.
Best regards,
-- Al