It's Python (a Django app). BSD license. It has many advantages over Wordpress that I won't get into here, but all this is offset by one massive disadvantage: It rests on the Django framework.
Django is a nice framework for web development but it takes at least a few months to learn for someone starting from scratch. The installation process for Mezzanine requires some Django knowledge.
Installing Wordpress on the other hand requires zero PHP knowledge. A reasonably intelligent person can teach themselves Wordpress basics in days (the basics of self-hosted Wordpress, selecting a theme, selecting key plug-ins, etc.). No PHP required.
With Wordpress, it's not too difficult to select a provider (i.e. Bluehost) and have Wordpress automatically installed within minutes using Simple Script. If Mezzanine could be automatically installed like this, I think it would quickly become a popular and much more maintainable alternative.