What Type of CMS Does Your Website Require?
When most people think of a CMS, they think of the top three that are commonly used: WordPress, Drupal and Joomla. Many tend to think of those CMSes as simple, and limited to specific purposes, limited to those users who are looking for open source solutions. Others will claim that using those CMSes, one could potentially develop any site, from overly complex corporate sites, to simple business sites offering products and services.
It breaks down to your definition of simple vs. complex. For a website with hundreds of visitors, a CMS that displays lists of users and content items would be fine. For such a site with hundreds of thousands, or even millions of users, such a list would be unusable. Other issues, such as scalability, content re-use, globalization and workflow all become problematic for large scale sites, and at that point it is advisable to consider additional Web Content Management research.