Does a demo have to be playable?

Hi!
That's what I am thinking about now. I want to put a "Lite" or demo version of my game on the internet, but I don't know what to include and what to exclude.
What will make someone want to buy a game, and what will be enough that they don't need to?
The way I see it there is:

• Core rules
• Rules clarifications
• Character creation
• Background setting
• Custimization rules

What can I include that will entice players to buy and what should I not include that will discourage players from buying?

Dave M
Author of Legends of Lanasia RPG (Still in beta)
Free Demo