Excellent foundation by being based on Debian. But with excellent long term support for security updates in the LTS editions along with a huge community providing an unparalleled pool of community knowledge.
A clean, easy to understand, well documented framework with excellent tools and a great community providing every imaginable extension to add functionality to your project.
By far the best free Analytics system without having to worry about self hosting.
Plenty of experience in using and customising the framework for use in multiple projects. Also its ubiquity on the internet makes finding good themes and support much easier.
Fantastic feature-set available for free comparable in features to the paid alternative (Github).
Even the free trial has proved useful. Enabling us to see potential UX issues in the use of the site.
Fantastic app and I hope they keep the free tier as its very useful. Its helped me fix a couple of problems before the user has contacted me. Its a great feeling to say - "Try again, its fixed".