Believe it or not, sometimes mongodb is the perfect tool for the job. It actually does support safe wrriting features, so some of the hype in that video is false. MySQL is always a great piece of the stack but it's not always good.
The video also shows its age, MongoDB hasn't stood still in the meantime. I put up a transcript of this movie because I found the mingling of the technical discussion with contemplating working on a horrible farm an interesting concept.
If I may toot my own horn, I just upgraded the layout of http://mongodb-is-web-scale.com/ so that you can scroll while the video stays visible.