10 MINUTES RULE

I found this interesting article about how the '10 minutes rule' could be applied to videogames.

Although the writer puts much effort to show how to do it from a narrative prospective, I think you can achieve the same results using gameplay mechanics: Showing some of your best gameplay features at the beginning, or simply creating an outstanding level design situation that makes the player feel 'Wow! I gotta keep playing this!'.

But anyway I do think you need a 'hook' at the beginning of the game or demo. Otherwise, considering the amount of leisure offerings that a standard user has everyday, he'll downgrade your product from the 'Must to' to the 'If I have free time...' and you're screwed.