CMS Differentiating Factors

Hello again,

I understand that there are something like 1300 CMS products on the 
market and only about 30% featureset overlap between these products.  
This would seem to make it almost impossible to select a CMS.  I am 
doing general research on this subject, but I am interested in input 
from experienced users, developers and administrators regarding the most 
important features which differentiate one CMS from another, from very 
generic things like support or enforcement for web standards and 
accessibility to very concrete and specific things such as support for 
XSL 2.0.  Feel free to include information about specific vendors, but I 
am mostly looking for more generic input.  I wouldn't mind if this turns 
into a general discussion on CMS selection criteria, though I'm sure 
that has been covered elsewhere.

Best regards,

   -John