When working with clients, I often heard a number of requirements that, in my opinion, had little to do with what a modern CMS really needs. Far too often emphasis was on features that should have been in the domain of template designers, such as support for responsive design (whatever that really means), the ability to add brand elements etc.
I compiled a list of the top 5 features a modern CMS needs at http://www.simoahava.com/content-management/modern-cms-top-5-features/
Modular architecture and strong security are the most important features from a technical standpoint. Complete control over content, source code and the software solution itself are all huge perks for any CMS.
Simo Ahava