The reward structure of video games can be the key to their success, or spell their doom. A reward is something that the game gives you for your work, this can be a new item, ability, story progression, or whatever.
In World of Warcraft the reward structure breaks down like this:
At Levels 20,40,60, and 70 you get new modes of transportation.
Every 10 levels, you tend to get an ability that changes the way you play, either through talents, or trained.
Every 5 levels, you gain access to a new tier of talents.
Every 2 levels, you get to train abilities.
Every so often, you go to a new zone.
Every level, you get a talent point.
Every quest, you get a reward.
Notice the spread here. You're constantly getting reward via quests. Every so often, though, you get a 'bonus' of sorts. There's always something in the horizon for you to pursue. Big rewards are few and far between, like the mounts and the play-changing talents, they don't come often, but they make the game feel so different and new.
Not only do rewards make the player feel good about what they have accomplished, but it gives them something to look forward to. When I'm playing, I always find myself saying something resembling 'I'm so close to X, just 2 more levels!'
But how does this system work once the levels stop coming? One issue with the original World of Warcraft endgame was that there never was any guaranteed reward. The only reward/progression of character came in the form of gear drops. Since gear drops were random, and multiple people vying for the same loot, it can take a long time before the player obtains the item they desire, possibly never. However, in the Burning Crusade and Wrath of the Lich King expansions, every boss drops Badges or Emblems for every player. These can be exchanged for items. This gives the player a timeline on their item acquisition. They can say 'Only 2 more emblems and then I can get X.' This gives each player a reward for their effort, independent of the random number generator.
Furthermore, the raids themselves are subdivided. Naxxramas has it's wings. Ulduar has its sections. So although you may not have cleared the entire raid, you can say 'Yes, we just cleared the Antechamber!.'
In Cataclysm, World of Warcraft will be introducing a level of progression that will only be available once you reach the maximum level, offering upgrades and rewards for players who can no longer level.
Another game that I like that has a similarly styled reward structure is Star Fox Command. Each playthrough for the game is broken up by planet. Each planet is broken up by turns. And each turn is broken up into skirmishes. Furthermore, playing through the game in certain ways will unlock new characters who have different styles of play. No matter how long you have to play, there is something that you can achieve. Combined with exciting yet varied gameplay, it makes for a wonderful game.
These 2 games offer a reward structure that is, in a way, jagged. If you were to graph it, they would look like mountain ranges: rough, with towering peaks.
Your 'typical' action game, such as Devil May Cry, God of War, Halo, etc. Have different sorts of reward structure. It's more flat, the peaks are smaller, but more frequent. Each level has an ends in a boss battle. Arsenal upgrades come every few levels, with new abilities and weapons. Each level is broken up into different skirmishes. The gameplay isn't varied, with each level being like a different skin on the same creature. In order to enjoy an action game, you have to really love the gameplay, and enjoy challenge. Fortunately, that's what the designers spend all of their time on.
Puzzle games, too, have this sort of reward structure. It's fairly constant, and you must enjoy the challenge and the gameplay in order to enjoy the game.
What's to be learned from this? I'm not really sure, but it is interesting to consider how different things in your life reward you. You could consider a television show that you watch, and how they dish out reward. When do the 'big' moments happen?