If I had my way, I'd put the ingredients for crafting specific magical items in the Monster Manual instead of in the DMG. For example, under the entry for white dragons I'd mention that scaled armour from dragons grants frost resistance, and swords quenched in the dragon's breath weapon when being forged are weapons that do bonus frost damage. Sure that might lead to some squickiness (ie. gloves of dexterity made from quickling leather, amulet of poison resistance made from a unicorn's horn etc.) but I don't mind magic being a bit taboo.
I think this method allows players to crafting magical items (and gives a reason for wizards to go adventuring), but doesn't allow you to create whatever you want, when you want. The more basic the magical item, the more mundane the ingredients. The more the powerful or game-changing the magical item is, the more exotic ingredients that you need.
Plus, like spell components, it is one of those things that can be a modular option that most people are free to ignore, but is fairly immersive for those who want it. Also, like spell components, just because you don't use this rule it still adds immersion by having it. The fact that sleep uses rose petals and sand to cast the spell in the spell description made the magic seem more alive, even if you never asked your mage whether he had any sand on him.
I also approve of the idea that +1 to +5 be a matter of smithing rather than magic. It gives something for smiths to do that isn't invalidated by enchantments, and allows for magical items to be improved. So you find a sword of frost in a dungeon crypt. However, while well made, the balance can be improved and the blade tempered by a skilled smith. Heck, maybe dwarves as a racial ability can improve existing weapons in a way that doesn't really seem possible to us humans (ie. cannot happen in the real world) because we don't know their secret arts.
