First of all, the equip ability didn't have mana costs. It still could only be played during your turn, and only on an empty stack (in other words, only when you could play a sorcery), but all equipment essentially had Equip . But that's not as fun as it sounds.
|"Ring of Invisibility,"
the card that eventually
became Whispersilk Cloak
While Equip is fun (and moving Lightning Greaves from creature to creature for free is, indeed, fun), it gave Mirrodin's development team few options for balancing powerful equipment. If equipping was free, then the cost to play the equipment had to go up. That left many fun equipment cards too expensive to see play; for example, the card that became Mask of Memory originally cost . So equip costs were born (and cards like Leonin Bola and Surestrike Trident, with special non-mana equip costs, became possible).
The other twist of equipment, early on? You could equip your opponent's creatures! Again, it sounds cooler than it really turned out to be.
|"Jagged Sword," an
equipment that generally kills
the creature that wields it
(did not see print, but inspired
The flavor of equipping your opponent's creatures is nifty: you have a cursed or hard-to-wield weapon that you stick onto a soldier of your enemy's rank, and the soldier suffers its effects. But in practice, this was too confusing, and didn't add enough design possibilities for the headaches it caused. Cards like Vulshok Gauntlets and Skullclamp still capture the flavor of "detrimental" equipment, and create additional value for equipment-stealing cards like Carry Away and Murderous Spoils.