This is an age old question in the Content Management world. In 2007, Deane at Gadgetopia came up with some really good answers. What's great is the list is broken down into essentials and nice-to-haves.

At it most basic level a CMS should:

• Accomodate information that can take many forms -- text, images, hyperlinks, etc..
  
• Enable users that are often non-technical personnel to manage site content.
  
• Facilitate content management as a collaborative effort that may involve various roles.
  

There are many definitions of a CMS. Here is the CMS defined according to typo3. A content management system (CMS) is a computer application used to create, edit, manage, search and publish various kinds of digital media and electronic text. CMSs are frequently used for storing, controlling, versioning, and publishing industry-specific documentation such as news articles, operators' manuals, technical manuals, sales guides, and marketing brochures. The content managed may include computer files, image media, audio files, video files, electronic documents, and Web content.