Let me start by explaining what I understand under the term replayability. This is a feature of the game, which talks about how much the player is inclined to return to the product even after it has completed. Under this concept I also pull games that do not have as such an end, eg Minecraft, World of Warcraft.
I will try to describe here the most important things that influence the replayability of examples.
History and an expanded world.
A really good story, and an interesting and powerful world in the game can be drawn like a good book or video. An example of such a game is Baldur Gate. He has countless quests and choices that the player receives. Many times I passed this game and each time the gameplay was to me in a different way. It depended on my character, from my choices, and even whether or not it happened in a certain place at a certain time. A similar effect is Planescape Torment and Fallout.
Non-linearity of gameplay
Gameplay is different every time, depending on the player's choice, or for random reasons. This game design makes the game look different every time, and puts other challenges to the player. An example of such a game can be the Silent Storm, in which the missions are selected randomly.
Player awareness that the game has several possible endings will certainly encourage him to delve after her again. It is important that the endings are not determined only in the final stage of the game, as the lazy player can save the game at the right time and check another ending, for example I can give FarCry3. A much better solution may boast of Bioshock, where the end of the game depends on whether Ratowaliśmy sisters or used them. Yet different to the end we can see the approach Planescape. In this game in the final stage we have a choice of several options, which depend on whether we have special items, or what are the statistics of our character.
A multitude of characters and classes
The best example of a game where we have several characters to choose from and each offers a different experience will be Diablo. It was possible to play a barbarian, which his opponents siepał wielgachnym axe, or sorceress favors to keep the distance to their opponents. Additional spice added skill trees. Another interesting approach is for example the game Jazz Jackrabbit, in which some locations, or paths are only available for the selected character.
Type of Game
This is probably the most important factor influencing replayability. Usually games with a theme of creating or growing something have a larger replayability coefficient. This is due to the fact that people like to create something or to watch it grow. A great example of a game where we build something is Minecraft. The game does not have a major goal. The goal of the game is to create and it just attracts the player. Nor do you mention The Sims game, where the player cares for the family and builds a home for them.
Some games each time offer completely different starting conditions. A great example is the series of Civilization games. Starting the game we can only define certain parameters by which the world will be generated.
Other factors affect the replayability of simple Tetris-type games. In this game you can not win. The goal of the game is to lose with the best result. The player can reach for this product for several reasons: it is trivially simple; It takes a long time; She wants to beat the record.
Mods and Editors
It has long been known that map editors allow to prolong the life of gryo many years. For example, let's use the Heroes of Might N Magic series. On the internet you can find thousands of great (or worse) maps for those games that can hold the man at the computer.
The second issue is fashion, which can add certain elements to the game, change them or remove them. For example, I give a fashion to add new adventures to the Baldur Gate. I also know that many players like to Modde games like Skyrim. Many times I stumbled on pictures of type 'too many mods '.
Online games have a completely magical power of attracting players to monitors for long hours. Some of them, such as WoW, require players to co-operate, making the ties between them their. Therefore, players have another reason not to leave the game, because they want to stay among their friends.
Many of these games makes the player feel that something has invested in the game and does not want to lose it. They don't necessarily have to be money. Usually it's about time or learning the difficult art of the game. For example, let us use World of tanks. The rules of this game are simple, but reaching the championship requires hundreds of hours spent learning opponents and maps. In addition, the player constantly develops his garage.
In addition to the factors influencing replayability, you should be aware of things that discourage a player to re-delve after playing. The main factors can be its weak execution, errors, crashes. Another downside will be poorly designed and unintuitive UI.
I realize that I did not delve into the subject even half as if I wanted to. It should be understood that the factors influencing replayability are as many as the games themselves and it is extremely hard to collect and describe them.
Some advice at the end: