Iron is the material composing a large portion of the items in the Dungeons of Doom. Iron can corrode or rust. Rubbing iron on a touchstone produces its signature "scritch, scritch." Hitting a brown or black pudding with iron splits it into two. Polypiling iron objects results in an iron golem.
Wearing any armor made out of iron will inhibit casting of spells, so they're generally not used among the wizard class.
In UnNetHack, elves do not regenerate health while touching iron with bare skin. Weapons may be wielded without consequence if gloves are worn. Similarly, amulets do not inhibit regeneration if worn over a shirt or body armor.