I think that's an important question. I don't know the answer as yet, but I find that I get closer to an answer when I speak to persons who use other CMSes or just watch them as they work.
Perhaps our USP has to come as a unique combination of features out of the box that you'd have to combine several CMSes to gain a comparable out of the box experience.
Features that I really value are:
Intuitive cut and paste
I really like the ability to cut and paste entire pages and folders and move things around, just like on the desktop. It makes managing the information infrastructure much easier.
File link integrity
Use case: Customer needs to rename a pdf that is available for download because the name is misleading.
Unfortunately you have 3000 pages other pages scattered throughout your site that reference that pdf. Luckily for you, you're using Plone so renaming the file (not just the title but the actual url) is safe.
Sane Group management
We recently benefited from Plone's built in group management capabiliites. It was easy to create three folders (A,B,C) and three groups (A,B,C) then give sharing permissions to the folders based on the groups. This was trivial to implement and train a brand new user. To achieve this on many of the other platforms would introduce additional "non-native" customisation which would require proper planning for migration and security considerations.
Security is not something to be trivialised, I've had to interact with administrators who deal with other CMSes. Getting hacked can be a huge mess.