Aligned weapons

In the 3.x era, aligned weaponry raised some logical questions since it meant that when angels and demons fought their damage reduction did not work against each other, yet when demons and devils fought their damage reduction did work against each other.

5e has dropped 3.x's convoluted damage reduction mechanic in favor of a unified mechanic for damage vulnerability, resistance and immunity. Aligned damage no longer exists, although some effects deal damage to particular types: an arrow of dragon slaying deals extra damage to dragons and otherwise harmless holy water inflicts radiant damage to fiends and undead.

I feel that removing the concept of aligned damage was unnecessary if alignment's place in the rules has essentially been replaced by types, and some monsters like vampires and werewolves still resist non-silvered weapons. Furthermore, the mechanic of extra damage makes it easy to derive different degrees of harm to provide more granularity and avoid logic gaps.

For example: fiends might be resistant or immune to damage from mortal weapons but not fiendish weapons, whereas celestial weapons not only ignore their resistance or immunity but deal extra radiant damage. In practice, this means that a fight between demons and devils would deal damage normally to both, but a fight between angels and demons would result in even greater damage to both.

So you can both use your xibalban blade to cut an otherwise invulnerable demon as well as splash it with holy water or brush it with a blessed dagger to cause burns.