MMOs have to work to influence the player's perception on quite subtle levels. When a player gets a reward of some sort, there's often a visual and/or auditory cue. It's good psychology to make sure that getting the reward is itself a rewarding experience, so that players feel accomplished as well as knowing consciously that their character is now better off.
Take, for example, the silvery ching of looting coins in, well, practically any game that you get to loot coins in. Or the pyrotechnic eruption, visible by everyone, when a character levels up in City of Heroes. Or the brash fanfare of a level-up in Warhammer Online. It's hardly coincidence that the universal slang for 'I just levelled up!' is 'ding!' - the bright, satisfying auditory signal that accompanies achievement. In short, games are full of flashes, whistles and bells designed to make you feel really good about what just happened, and more importantly, make you want to do it again.
But which reward effects, major or minor, are the ones you like most? This blogger has a soft spot for the level 50 'ding' in City of Heroes, which has an accompanying fireworks display as well as the usual light show. Are there any that strike you as really satisfying? Conversely, are there any flat let-downs that really don't deliver? (Age of Conan leveling, I'm looking at you...)