More on special items, inspired by another Gameplay Classic, The Legend of Zelda: A Link to the Past.

Using items to pass boundaries

RPGs usually have soft and hard boundaries woven into them, to keep players on a more or less certain path. Soft boundaries are things such as level, loot, or coins. The players would have to invest a certain amount of time or work before they could reasonably progress in the game, i.e. beat a boss or traverse a higher level area without getting killed on sight. They are called soft boundaries because players can get past them eventually, maybe even earlier than the intended point of time in the story. Hard boundaries are impassable cliffs or invisible walls or story related blocks (the town's gate is closed because -insert arbitrary reason here-) that keep players where the devs want them to be.

Zelda featured objects in the game that were hard boundaries, which changed into interactive elements once you acquired particular items.

An example: You'd have boulders sprinkled all over the game map that block paths. Some of them would block interesting places you want to get to. You'd wonder how to get there. Once you acquire the Power Gloves in one of the dungeons, you can, from that point onward, lift all white rocks all over the game map, allowing you to get past them, opening up new areas, shortcuts or secrets. It's not a one-time thing either.

Most of the items in Zelda had a special gameplay function that were, once found, used throughout the game (bow to activate far-away switches, harpoon to haul in items across a distance, lamp to ignite torches or navigate in completely dark rooms, bombs to blow up cracked walls).

Now, I realize that in an RPG those functions would probably be replaced by skills and spells. But still, the boundaries themselves were refreshingly different from story- or engine-related boundaries we usually find in RPGs, making the game world more interactive in a fun way.