Look, I've worked with Microsoft SQL Server, MySQL, Oracle and PostgreSQL in total now for 11 odd years (production experience, excluding studies) and I'm astounded at the versatility and SPEED that MongoDB offers. I'm buying into the NoSQL movement wholeheartedly as I can see the benefits it has for developers and support technicians in the long run.
Remember that MongoDB is fairly new technology, but it's being backed by industry giants currently. Many of the initials issues it had has been ironed out (like database-wide write locks, now down to collection or document-level locks) and they even started implementing features that will make traditional RDBMS fans feel more at home, like "joins" ($lookup) and references (DBRefs).
In essence the 2 technologies only have one feature or use in common: persisting data. The ways in which they operate differ wildly so it is futile comparing the 2 technologies. Heck, it's even futile comparing SQL vs NoSQL databases - this are just too different from one another. What I do agree with [MENTION=2226]InSanity[/MENTION] with is that it is situational, as is the case with any piece of technology. In environments where you need high performance (think high transaction volume web applications or transaction engines), have highly volatile data structures (workflow systems are a great example) and would benefit from implementing only one programming language in your application stack (in this case JavaScript), MongoDB is almost the default choice.
I'm a
huge JavaScript fan and evangelist, but I don't think that makes my opinion biased at all. In contrast, I've been a fan of Microsoft's stack of technologies (Windows, ASP.NET, MSSQL, IIS - let's see if the term WAMI can take off) for my whole career and with recent experience in the Oracle stack (Linux / Unix, Java EE, Oracle, Glassfish) I've also become a fan thereof. With each of those stacks, you typically end up developing in 3 languages when you build a modern, rich web application: C#, T-SQL and Javascript; or Java, PL/SQL and JavaScript. The MEAN stack reduces that to a single language: JavaScript.
I'd really urge you to follow the current development trends in the industry and notice how large corporates are adopting the MEAN stack. I followed the hack.summit() 2016 this week and was surprised to see how industry giants like Microsoft, IBM, PayPal and SAP are all buying into, supporting and improving Node.js. Just browse through MongoDB's site to see how many tech companies have already adopted the platform in just the last 2 years (E-bay, LinkedIn, Adobe, EA, New York Times, etc). Now is the ideal time to jump on the JavaScript bandwagon - it's set to make big waves in the industry in the next couple of years and it will finally break the stigma that it's only useful for validating input fields on a web form.