Quote from MTops
This is true for skills that have no damage type of their own. Spectral Blade is about the only Wizard skill that has no damage type, so it copies the one from the weapon. For example, Electrocute will always show the lightning death animation.
Quote from Sabvre
Wave of Force
Actually these are both considered Physical dmg I believe. Any physical damage skills will copy the elemental dmg effect of the weapon. Something like the Witch Doctor's Poison Dart will always appear as poison damage obviously, however it remains to be tested if the elemental damage of the weapon still carries through - just that we don't get to see it.