A track record. When I've donated to or pre-ordered games, it's always been because I've played and enjoyed previous games by the creators. Partly it's that a track record of successful, completed projects gives me confidence that the game will be finished and released (and reason to hope it will be good), and partly it's an appreciation of the games I've already played for free, so that even if it falls through it won't feel like I've thrown the money away.
For example, I donated to
Resonance because Vince Twelve's previous games,
Anna and
Linus Bruckman, were some of my favorite AGS games. And from the bits I've seen of it, that investment (though it has been a long time to wait) was absolutely a good one.
