while i never played in an edition where it was a really significant factor i really love the concept of there being high-tier materials to make weapons and armour from that all had their own special properties and picking what to have your gear crafted out of would have significant impact on how you used it, mithril being super lightweight so you can make heavy armour count as medium like was mentioned or put thrown on bigger weapons, maybe oricalcum being anti-magic so you have magic resistance while wearing it or its weapons deal damage to normally resistant creatures, adamantium being superstrong making damage reduction armour and weapons with upped damage die sizes, and then there are the classic silver and cold iron.
there's so much you could do with a proper material system.