With the release of version 3.0 and with it Custom Post Types, WordPress took a huge leap forward as a CMS. However, the full potential of Custom Post Types and Custom Meta Boxes is not yet being realized because WordPress doesn't provide a default UI for adding post types and meta boxes.
There are indeed various plugins that try to solve this problem (magic fields, custom field templates, ...) and most of them do it fairly well. But in order to make these new features more accessible and reliable, there should be a UI that is part of the default WordPress installation.
Just look at Drupal and the integration of CCK into the core of Drupal 7. WordPress could do the same thing and become an even better CMS!