Game Designers can make there games have more replayability with HighScores, Achievements, New Content, Patches, Alternative Endings, and Mods.
Pac-Man is a very basic example of re-playability. HighScores are tallied at the end of the game. People always wanna beat their highscores. If they didn't exist pac-man would still be cool but not as fun and enduring. HighScores are even more fun when there are more than 1 person playing the game inside the house. Beating your brother or sisters highscore can be fufilling.
Achievements make players feel good about what they have done. Look at xbox for example. People obsess over achievements. They will play a game as many times as possible to get all of the achievements. This is a very good thing when it comes to replayability. Achievements are being overused too much though. Ecverygame shouldn't have achievements. Only games where a multitude of scenarios can happen in a given instance should have some sort of achievement system. Pac-Man would be a poor candidate for for an achievement system.
Patches and updats are a great way to update an already awesome game. Peggle Nights is a perfect example of this.You can download a christmas level pack from popcaps website. Its pretty awesome. And you can download it for free. Free stuff is always a good insentive.
World of warcraft probably has the greatist replayability of them all due to its extensive patches and updates. This can be a problem though. The game never ends so players are hooked for way to long. Games are ment to be a break from everyday life not life itself.
New Content(Bonus Levels, New Abilities etc)
In single players games unlocking new content is always fun. Beating the game over and over to get new things can be a blast. Especially when you get stronger powerups and weapons to greaten your highscore and or rank.
Beating a game once may not be enouph. Some players want a diffrent outcome. Fable does a perfect job of letting the player choose their endings. This ads a heck of alot of replayability.
I personally played fable alot of times just to see how diffrent the game would be if i went the bad path or the good path.
Mods are very popular for pc users. Most games on the pc can be modifeid and tailored. This is because the files are readily available to players to tamper with. Some game companies like unreal actually dedicate an engine for players to create real levels and even games. Mods add a whole new dimension to replayability. Not only can you modify the game but you can play levels never before seen that where created by players.
If you allow players to add their own content to the game then it gives them a sense of connection to the game you made.
What if my game doesnt have Replay Value?
***Great Story Lines & Sequals ***
Not all games have great replayability, but a common fix for that is sequals. Once players beat a game with a great storyline they want more. My roomate played kingdom of hearts over 7 times. I personally beat spyro 3 over 7 times as well. If you can get players to love the storyline then they will be looking forward to a sequal. That's the next step if you are having trouble creating replayability within your games. Make the game fun so players wont mind doing that same quest over and over. And make sequals to your games.
Make sure your game has atleast one of these tricks so that your game can be playable and Replayable :-)