An interesting article about how Diaspora starting using MongoDB and switched over to MySQL after they ran into problems with consistency. It's a little long but I would recommend reading it because it explains one of the problems with using a document-oriented database. I would also like to say that I'm not a NoSQL database expert but I love the structured nature that traditional SQL gives us plus we have decades of experience with people using it.

http://www.sarahmei.com/blog/2013/11/11/why-you-should-never-use-mongodb/