Open content web sites depend on users to produce information of value. Wikipedia is the largest and most well-known such site. Previous work has shown that a small fraction of editors - Wikipedians - do most of the work and produce most of the value. Other work has offered conjectures about how Wikipedians differ from other editors and how Wikipedians change over time. We quantify and test these conjectures. Our key findings include: Wikipedians' edits last longer; Wikipedians invoke community norms more often to justify their edits; on many dimensions of activity, Wikipedians start intensely, tail off a little, then maintain a relatively high level of activity over the course of their career. Finally, we show that the amount of work done by Wikipedians and non-Wikipedians differs significantly from their very first day. Our results suggest a design opportunity: customizing the initial user experience to improve retention and channel new users' intense energy.