--Comprehensive image library handles all types of media (photos, audio, video)
--Flexible navigation heirarchy
--Designed for ease-of-use for the admin
--Lots of jQuery integration (sliders, tooltips, autolookups)
--Templating system built upon 1KBGrid
--Automatic detection of mobile devices to allow switching to another version of the site
--Clean URLS help get great SEO placement
--MVC architecture with local folder override so core code doesn't get disturbed
They love the front-end look and appreciate the admin area ease of use. Great SEO placement doesn't hurt either!
Code is open-source and easy to read, well commented.