The loot economy is just as important in game design as the physics system, the weapon sandbox, and the ui. A deft touch on the "throttle" is required. Players interact with games in different ways at different times so all the aspects of game design need to work together to make the game "fun". Some people get the rush from completing difficult content some people get it from aquiring a choice piece of loot. It's all valid as long as they're feeling rewarded. Ideally it's a balance of all the systems working together that make the game fun.