Replay value
Replay value or replayability is a term usually found in combination with video games, but it may be also used to describe other kinds of games, movies, music, or theater plays. Video and computer game players use the term replay value to describe the entertainment value of playing a game more than once. In some cases, factors that influence replay value are the result of the game's programming (extra characters, alternate endings, etc). Other times, the replay value of a game might be based entirely on the individual's tastes; a player might enjoy a game because he/she likes the music or graphics, finds the gameplay entertaining, or (in the case of licensed products) because of loyalty to the product line. Generally speaking though, a game with dynamic environments, challenging AI, a wide variety of ways to accomplish tasks, and a rich array of assets will keep a player coming back for more.
Factors that can influence replay value
Plot
A game with a linear plot will typically have a lower replay value due to the limited choices a character can make. Games that offer more choices in regard to what the player can do, such as strategy games, roguelikes or construction and management sims, tend to have higher replay value since the player might be able to make each playthough different. In the case of role playing games, plots can be linear or non-linear. The plots of older RPGs tended to show little to no plot changes with each playthrough. A good example of an RPG with a linear plot is Final Fantasy IV. Role playing games released in later years allow for more freedom. For instance, choosing to be evil instead of good might prevent certain events from taking place (or even cause new ones to occur). Likewise, allowing a particular character into the party could cause the plot to branch off in a new direction, if even for a short time. A good example of this point is the character Juhani in Knights of the Old Republic. The player has the option of killing her or sparing her life. Should the player be merciful, Juhani joins the player's party. Later in the game, the player will then run into an enemy from her past.
Multiple classes
This is the ability to play as different characters each time through the game. For example, the game Diablo allows the player to choose one of three character classes: warrior, rogue, or sorcerer. Party-based RPGs can have a good degree of replayability, even if the plot is essentially linear. The plot of the first Final Fantasy game remained the same with each playthrough, but since the player was able to choose his party of four from six different classes he could choose a new party configuration with each new game. The Final Fantasy instruction manual even gave the player ideas for different types of parties (well-balanced, heavy magic use, combat oriented characters, etc.). In the first Final Fantasy game you can choose from the classes of Fighter, Thief, White Mage, Black Mage, Black Belt, and Red Mage, revising you with possibly 36 possible character combinations. (6X6)
Note that the characters change class as you progress: Fighter-Knight; Thief-Ninja; Red Mage-Red Wizard; White Mage-White Wizard; Black Mage-Black Wizard; Black Belt-Master.
Multiple characters
In some RPGs, such as Skies of Arcadia, the player controls the leader of a party of characters, and additional members will join the group if the player makes the right decisions. It is unlikely that a player will "collect" every possible member on their first attempt, so they may choose to play through the story again in order to acquire the full cast. Different characters offer new ways to tackle the obstables within the game. In the case of action-oriented games, there may be some areas in the game that are only reachable using a character with certain abilities. For example, see the Sega game Sonic & Knuckles. The character Knuckles can explore areas in Sonic the Hedgehog 2 that the previous characters couldn't.
Alternate Paths
Some games give the player a choice of which path they want to take through the game. A good example is Castlevania III. In this particular game the hero Trevor Belmont can choose which path he takes to Dracula's castle. Additionally, the route he takes allows him to encounter three different companions.
Unlockable characters and content
Sometimes beating the game or completing certain challenges will allow the player to use an NPC from the game or even a new character. For instance, in Baldur's Gate Dark Alliance, the drow ranger Drizzt Do'Urden is an unlockable character. Other examples of unlockable content can include art, music, behind-the-scenes featurettes, and interviews with a game's producer, artist, or voice talent. Electronic Arts' games based on The Two Towers and The Return of the King feature many different unlockables of this nature. As the player progresses through the game, he can view production stills from the movies, concept art, and interviews with some of the actors (including one where Ian McKellen laments that he never plays video games because he can't understand the rules and lacks the required hand-eye coordination). Completing either game unlocks new characters, missions, and cheat codes.
Alternate endings
One of the most common ways to increase the replay value of a game is to offer multiple endings. These endings are often based on the choices the player makes during the game, like choosing to be good or evil. Several games based on the Star Wars series make use of alternate endings, such as Knights of the Old Republic, Knights of the Old Republic II: The Sith Lords, and Jedi Knight Jedi Academy. The endings for these games are based on whether the player chooses the Light Side or the Dark Side of the Force. Marvel Ultimate Alliance is another game with multiple endings. During the course of the adventure the player will need to make decisions (such as choosing whether to save Nightcrawler or Jean Grey) or complete certain tasks (like stopping a group of mutated soldiers from destroying a computer that holds research on the Legacy Virus). After completing the game, Utu the Watcher will give a glimpse into the future and tell the player what the consequences of his actions are.
Of note is how The Knights of the Old Republic games make use of several of these factors to increase replay value. In the first Knights of the Old Republic game, the player chooses one of three classes: soldier, scout, or scoundrel. Later in the game the player chooses one of the three Jedi classes (Guardian, Sentinal, or Counselor). Thus, the player has nine possible character combination choices. In Knights II, the player starts as one of these three Jedi classes and gets to choose a prestige class. Additionally, the player gains different followers depending on whether he selects to be male or female and Light Side or Dark Side. Both games allow the player to customize his characters by selecting various skills and feats. Thus, a player might choose to focus on two-weapon fighting for one playthough and single-weapon fighting in the next.
Online gaming
In massively multiplayer online games and other types of online game, the other human-controlled players give the gameplay a greater variety than permitted by the AI of computer-controlled bots, as well as allowing chatting and other interaction between players, thus increasing the length of time the player will spend on the game.
Other factors
Games where the map and starting position is different every time you play it, in for example strategy games with a random map generator, also tend to have long-lasting appeal. The community-developed mods for many games also contribute to increased replayability and long life for many games. Due to the limits of AI behavior and the appeal and challenge in playing against friends, multiplayer is also often considered to increase the lifespan of a game. For example, Worms Armageddon, a comparable simple and old (1999) game, is still popular online.
It should be noted, however, that variety in gameplay is not essential to replayability. Many arcade games, especially from the golden age of arcade games, are completely linear, yet people continue to play these games, even those that have been around for decades. A clear example can be seen in Tetris. The gameplay is extremely simple compared to more modern games; yet it is arguably one of the most addictive video games ever made.
Generally, games with time attack or time trial modes offer the highest replay values. This is because dedicated players can always aim to improve their times.
Many games, including The Sims, have unlimited replay value, simply because there is no way to win the game. Such games are open-ended and have no storyline or finish.