You probably know Peter Molyneux's name if you've been playing video games for any significant amount of time -- he's the mind behind such classics as Populous and Dungeon Keeper, all the way up to Black and White and the current Fable series. He recently gave a talk to the British Academy of Film and Television Arts, and early on his talk (part 1 is here, part 2 and part 3 are also online), he speaks out about our favorite game, World of Warcraft. Specifically, he mentions it as an influence on his game design, and says the most brilliant thing about the game are "the steeds," or the mounts you could pick up at level 40 (nowadays, of course, they're available at level 20). He says that in his own games, he tries to give everything out to the player as soon as possible, but the fact that Blizzard made you wait to ride a mount around, made you work up a few levels for it, really stuck with him.
Now, of course, he's taking away his own lessons here -- Blizzard's philosophy with the game as a whole seems to reward the player as much as possible, and especially lately, with emblems and the different modes and all of the other daily and weekly quests they've come up with, they're making you do less waiting for prizes than they ever have before (in fact, compared to MMOs when they first started, much, much less waiting). And Molyneux's own games are very "rewarding" -- I don't think more than two minutes went by in Fable without me getting a level or a new spell or a new item to play around with. But his point is still good, even after all that: anticipation of a reward can be just as strong a motivator as the reward itself.