Game Design: Getting the player to keep playing: Adding replay value

How to add replay to a game:

-More content
This can be done two ways:
1.Large amounts of content, which a player must go through twice or more if they want to see it all. This is done in Need for Speed Carbon, the player needs to continue playing or start again if they wish to see all cars, or to customise a large amount of time. This also to an extent is adding such a large amount of content that the player starts fantasising over what he could make, basicly giving the player the illusion that your game is is a Mary Poppins bag of content.
2.Content Exclusive to second+ run through, which is done in Fatal Frame 3. Basicly, the player needs to go through and see more. 'Nuf said.

-Achievements
By giving the player goals that extend beyond the first run through, we all see it alot, so you understand :)

-Procedural Content
The content is generated as you go along. So the landscape may be different, new looking enemies or new looking guns.

-Experimental Nature
Spore does this, Falling Sand does this and so do many games. What I mean is the player must keep playing, see what he can make and what he can see. This goes nicely with the other points I have made.

Ill try to put something cool up tommorrow :)

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