Calling it a CMS is probably incorrect. It is a tool for creating a web site. Unlike Drupal, ocPortal and such it is not a tool to allow you to have the unwashed masses contributing to the site.
The usual complaint about Wikis is that the syntax gets in the way. That is, you have to insert syntax (which is usually somewhat different in each Wiki) to get things done. Well, Zim has no syntax or, more accurately, you don't work with it or ever see it. Zim just uses it internally to offer the functionality. While you can't really call it a WYSIWYG, it's close and you don't have to do funny stuff to get the WYSIWYG features.
Did you ever use Wordstar or PCWrite?