Some people hold that Wikipedia is a good place to get started but lacks the rigor and reliability to make it useful for going more in depth. Having spent 15 years editing Wikipedia, I fundamentally disagree with this assertion. Certain articles can be very good (though not without problems), but the majority has very serious issues and can be extremely misleading.

Smaller topics are highly susceptible to distortion because there's less oversight. The Wikipedia articles on most mango varieties are a great example. Who thought mangos would be a contentious subject? But people are constantly editing the pages to push nationalist agendas: this variety of mango is from India. No, it's from Pakistan. The variety cultivated in my village has the highest vitamin content. And so on. These unscientific edits can endure for a very long time because more established editors and moderators aren't checking them.

So that means the larger, high-profile articles must be of higher quality, right? Well, sort of. They are generally better written, and blatant vandalism gets removed more quickly. Unlike smaller articles which can have bad edits that go unnoticed, moderators and power users watch the largest and most contentious articles like a hawk. But this is a problem, too. In recent years, Wikipedia has become increasingly controlled by a small number of power users. These are people who spend incredible amounts of time on the site and make tons of edits. They are not academics, nor do most of them have specialty training. They are just extremely online. And because they're so active on the site, they can control it by reverting edits, making long arguments on Talk pages, and skillfully navigating the complicated and insular bureaucracy of Wikipedia. This functions very effectively to prevent academics and professional historians from effectively shaping Wikipedia pages.

Imagine your college history professor noticing something wrong on Wikipedia. She's incredibly busy, but she decides to take a minute to correct the issue. The first barrier is dealing with Wikipedia's formatting and markup language, which is confusing. If she's tech-savvy, she can probably figure it out, but if she's older and not so skilled with computers (like many brilliant, tenured professors) that might be enough to deter her. Let's assume she manages to figure it all out and successfully makes her edit. Most likely, she feels a small sense of satisfaction that she was able to contribute something, and then moves on with her life. Sometimes, the story ends there, but just as often, it doesn't. At any time--and sometimes within just five or ten minutes--her edit could be reverted or otherwise undone, and she'll have no idea about it.

In order for her edit to endure, she'll have to keep watching the page, defending her edit from other Wikipedians who disagree--not to mention bots, trolls, and vandals. On Wikipedia, no one cares that she has a PhD, or that she's published in prestigious peer-reviewed journals on the subject. Since anyone can pretend to be an expert, the Wikipedia policy is to ignore all credentials. She therefore has to waste her time arguing with people who know less about the subject than the first-year undergraduates in her classes. And that time of hers is precious, because she works around 60 hours a week between teaching, office hours, faculty meetings, serving on various committees, mentorship, answering a neverending slew of emails, serving as a peer reviewer for others' work... not to mention doing her own research!

For these reasons and many, many more, people with actual expertise become massively discouraged from contributing to Wikipedia. In other words, people who have real careers and real lives don't have the time to succeed on Wikipedia. The inverse is also true: NEETs and people who otherwise spend all of their time on the Internet completely dominate the site, and in many cases it reflects their insular communities and worldview. These are the people who have the time and energy to win the edit wars.

So your college history professor doesn't have an impact on Wikipedia, but that guy from your computer programming class who spends his whole life online? He's a powerful moderator on the site.

These are actually only a couple of the issues; there are many more. Hopefully this gives you a sense of why Wikipedia is not only unreliable, it is fundamentally structured in such a way that it actually rejects expertise.

There's always more than can be said, but a similar question was asked last month.