Why work on Wikipedia? For me, the answer is a matter of scale. As a librarian, I am in the business of helping make sure that people get the information that they are looking for in order to do their jobs, educate themselves, satisfy their curiosity and live a fulfilling life. I am also in the business of helping people discover relevant information towards these ends that they don't realize or imagine exists. Wikipedia -- meaning the collection of people that produce this site -- is also working towards these goals, but on a global, multilingual and hitherto unprecedented scale. Because of the very heavy use the site receives, the changes that you or I make to Wikipedia are likely to touch substantially more lives than any other possible way of contributing to the information universe at this moment. It's a simple matter of efficiency -- I work on Wikipedia, and try to make it better, in order to reach as many people as possible.
There are other reasons as well why this is a deeply important project: the Wikimedia Foundation projects represent one of the most diverse global online communities around. The projects provide a way to get to know people from other parts of the world, and learn about their similarities and cultural differences, in a way that is unmatched online. The Foundation and Wikipedia also represent projects that are perhaps more comprehensively volunteer-driven and volunteer-governed than any other similar undertaking; the projects provide a model for what other empowered collaborative undertakings could look like and achieve. And finally, the sheer scope of Wikipedia is unparalleled in history. There's never been a reference work like this before -- never one both so general and so detailed, one that tries to be all things to all people in all languages. Wikipedia's existence is due to an accident of being in the right technological place at the right time, but it now affords a chance to work on one of the grandest undertakings ever.
Wikipedia is generally a friendly place, but it's also filled with arguments, disagreements, and actions that are angering or upsetting. For the most part people work out those differences through peaceful processes and resolution, with profound instances of assuming good faith; occasionally this doesn't seem possible. Generally, though, the Wikipedia communities are filled with some of the most extraordinary people I have ever had the pleasure to meet -- sometimes in person, sometimes not. I think most contributors who have spent very much time on Wikipedia realize what a cool project this is, and what cool people work on it -- but this doesn't get articulated enough to the world at large; it's easier to criticize a project than defend it well, and more importantly, to improve it. Wikipedia does of course have many areas in which to improve -- accuracy should be checked to a much higher standard, the rate of referencing is appalling, we can improve the climate for new and expert contributors alike. But I'm optimistic on all of these fronts, and hope to continue discovering beautiful and extraordinary evidences of human cooperation here.