If you want to supplement the idea of cold iron, then also saying that enchantment makes the weapon more compatible with the Fey metabolism would make things interesting. Sure your +3 sword hits better, but your regular dagger does +10 damage. Which do you use?
Ooh, I like that. So "cold iron" is any kind of non-magical metal weapon. Magic weapons, non-magical wood weapons, and claws etc don't work.
Of course, the Cold Iron Weapon enchantment from AV is the exception that proves the rule.