(Source: "Ecology of the Rust Monster". Dragon Magazine #88, 1984)
The thing that makes the Rust Monster an Icon for me is that its more than just a combat encounter, its a trap and a puzzle at the same time. How do you deal with a creature that will eat your precious +2 Sword or your shiny new Plate Armour? And unlike other monsters, an encounter with a Rust Monster will impact on future encounters because its will materially weaken the adventuring party in a way that no other encounter does.
D&D is largely a game of acquisition; the accumulation of XP, of Gold and of Magical Equipment are central to the players progression. So when a Rust Monster ambles around the corner of a dungeon it instantly threatens the players in a very personal way. It taps into the Psyche of the game and the players in a way that no other creature does and turns a simple scavenger into a classic monster.