Also, a good digital cable will use silver or silver plated conductors, the surface condition of the wire is very important (hence the plating being common) as the signal is high enough frequency that it will stay near the surface of the conductor (skin effect)
Impedance is the sqrt of L^2 + C^2 + R^2, R is usually small and L and C depend on physical construction.
As far as subjective SQ, a cable that is very accurate for analog signals generally sounds better for digital as well...
