Scott Adams in Rewarding Work:

Whenever you see the x-factor in someone’s output — that little extra something that turns the good into the awesome — it’s a marker for intrinsic motivation. Monetary motivation plateaus at the point you think your work equals your pay. For most people, that happens when the product is good but not awesome. To get to awesome you need to think you might be changing the world, saving lives, redeeming your reputation, attracting the mate of your dreams, or something else that is emotionally large.