Secondly, in single-player components, you should provide a "social" context for a reward, where society means either the player culture, the developer, or the in-game virtual culture. DON'T drop a bucket of gold out of nowhere for any achievement. DO place motivation behind that bucket of gold, show who gives it and why, what sacrifices they made, and how the player earned your quest giver's gratitude.
Also, DO give rewards that reflect and acknowledge player actions. Slaying a hard boss should reward combat prowess, while exploring a secret spot should give a bit of lore or a shortcut. It sounds obvious, but oftentimes reward schedules are so rigid that the progression seems dialed in.
Bad adventure games, for example, are guilty of not being cognizant of a player's accomplishments, rewarding random clicks or combinations of inventory items with unrelated plot continuation. As a puzzle game lover, my first test to see if one is good is to spam random moves, and see how much I score. A good one, like Tetris or Wetrix, can't be won this way.
Although Xbox Live's Achievements aren't a game in themselves, it is fascinating how they are just as addictive as if they were. They provide a meta-game between people that have driven most I know to play in whole new ways. Even when we aren't comparing our Achievement Points with others, we will treat them as a direct challenge from the developers.
Pleasure, though, requires no social context. I can get pleasure from the snapping of puzzle pieces together, the stomping of a goomba, or the realistic sound and animation of my shotgun decapitating aliens in Gears of War. Pleasure is a necessary component to addictive gameplay, but in isolation, it is merely empty calories.
Finding #2: People suck at predicting their future enjoyment.
Reason: Humans have very subjective memories great at recalling essential information quickly, but at the expense of retrieving unreliable details. Studies have shown that a person's feelings greatly influenced by both faultily recalled memories, and also their current state (known as presentism).
For example, people asked about how their overall life happiness was tended to respond positively on days their city had good weather, and negatively on days when the weather was bad. People displace their happiness estimates into their present state.
Applications: Actively predict and meet your players' expectations. The reward you dole out will be interpreted relative to the player's state at the time you hand it out. Giving an amazing reward when they least expect it will blow them away, but giving them pittance after defeating Beelzebub himself is a good way to earn the player's disgust.
Also, try to predict what kind of reward your player expects. Sometimes it's a shiny sword, sometimes it's plot progression, sometimes it's acknowledgment. You don't have to artificially lower expectations to improve your reward, just get them the right reward.
In Zelda: Phantom Hourglass, rewards are justified by making them immediately necessary to progression.
Put your fickle players in the mood to accept what they get. When Metal Gear Solid 3 put a narcoleptic old man with a group of elite assassins, I was astonished, yet somehow thrilled that he outsmarted and killed me. I shared Snake's lesson in humility, realizing the game had anticipated my expectations of an easy fight.
Don't feel you are beholden to your exact promised rewards. You may have told the player exactly what reward they can expect for an accomplishment, but their frame of reference will change when they are standing before your treasure box.
If it was a boring stroll to get to it, then the jewels promised inside can just be cubic zirconia. But if they beheaded a thousand demon bikers to insert the Key of the Gods into the lock, their definition of jewels is going to be very different. Just don't betray their mental image of what their goal accomplishment will look/feel like.