I discussed this a long time ago in a comment: <a href="http://news.ycombinator.com/item?id=226688" rel="nofollow">http://news.ycombinator.com/item?id=226688</a><p>The reward treadmill is the secret to retention. Get 'em hooked with quick rewards in the early part of the progress curve, then string them along at the highest levels. Otherwise people will quit in droves before the expansions come.