I thought that Schiff hit upon Wikipedia’s greatest weakness in her article “Know it All” when discussing the case of how William Connolley, an expert on global warming, ended up losing a shouting contest on Wikipedia against an opponent with little knowledge of the subject material.
People who edit Wikipedia for purely malicious reasons (aka “Trolls”) are far less of a danger to accuracy than those whose intentions are good (or at least not actively bad), but are misdirected. With Trolls, a patient expert editting Wikipedia simply has to out-wait juvenile attempts to spread false information for fun (which is hugely annoying, but also usually quite obvious and easy to revert). People who are spreading misinformation because it is what they truly believe, however, can be very difficult to deal with.
Part of the reason this is more problematic for Wikipedia than it is for a more traditional encyclopedia is because it biases vary greatly from page to page. Of course, older encyclopedias are frequently biased, as well, but usually these biases are consistent and can be determined, and so a reader can be conscious of such biases when they appear. On Wikipedia, there is no such consistency. Of course, this means that no single bias rules the encyclopedia- which is good- but it is also very hard to tell what political or social message any given page supports.