It's obviously a matter of some controversy as to whether WordPress is a CMS, but like it or not, it's an extremely popular platform.
Assume for a moment that it's an almost-there CMS.
What plugins would you add to the package to bring it over the threshold? ie, if you were designing a "WordPress CMS core", what plugins would you add?
Check out http://www.noupe.com/wordpress/powerful-cms-using-wordpress.html. It has a lot of great tips and resources for makeing WordPress a CMS.
Actually you don't need any plugins to use Wordpress as CMS. But I used the following plugins when I built CMS, they were specific for my site: qtranslate, cforms and exclude pages.
FYI WordPress 3.0 is coming out very soon, (it's in beta) and it adds a lot of API for adding custom post types (akin to Drupal nodes). You can actually do it already in WP 2.9, but 3.0 makes it easier to do.
Beyond that, what do you need to make it a CMS? Pages are quite flexible if you use templates. The answer to your question depends a lot on what you want to do in particular.
I recently did a Page-only site (no "posts" section) using just the PageMash plugin. PageMash lets you organize your pages easily (put them in order), and it lets you hide certain pages. So I set the "posts" page to a particular page and then hid that page via PageMash. Create all the pages you like and organize them at will. Works nicely.
Since you probably use a lot of regular pages on "CMS" web sites I always install http://wordpress.org/extend/plugins/cms-tree-page-view/
I am the one who created it, but it's true that I always install it and often I feel that I could not manage a site without it.