Simplicity of use and moderate learning curve.
It is a very mature product (more than 10 years)
Easily create and manage VM (KVM) and containers (LXC) through a comprehensive and intuitive web interface that is constantly evolving.
Support for multiple storage types: ZFS, NFS, CIFS, LVM, iCSI, Cepth, GlusterFS... and more
Comprehensive virtualization solution: Role-based management, backups, storage virtualization, firewall, high availability (Cluster), REST AP,I templates, live migration...
Free support via the official forum is very effective.
Very well documented through a very complete wiki, the latest versions have documentation integrated in the web interface.
The web interface is the best thing about this product, it is designed to be easy to use, without hidden menus or excessive clicks. VNC management of virtual machines is also done via the web, using the non-VNC client. If you want to enjoy a desktop experience on the VMs you can easily use SPICE.
Completely virtualize the entire HW and SW infrastructure of a department of my company.
This allowed for hardware independence, resource consolidation and keeping up-to-date backups of all virtual resources.