100% Completion

100% Completion challenges involve obtaining every item/trophy/achievement etc. and/or completing every mission or challenge within the game. Quite often these challenges can be long and tedious, but play on the gamers mind that there is something missing that they have not yet completed. Quite often there is a reward for completing everything, whether it be an achievement item, alternate endings, or sometimes just the satisfaction of finishing.

Examples
Games where this is common include the Grand Theft Auto Series, Pokèmon Series, some of the Super Mario series, and some of the Castlevania series, among many others.