Thanks to Steve Krause for finding blogHelper's Using WordPress as CMS. Steve notably omitted the counter from blogHelper, 5 Reasons Not to Use WordPress As a CMS. As blogHelper points out,
You’re Pushing It
WordPress was built to be a blog platform first, everything else second. So, everytime you push it to do something else, remember that you’re making it do something it wasn’t meant to do.
I see this all the time. People using WordPress, or more often MT, as a web publishing tool to try to build a website that really needs a full-featured CMS such as Drupal, Joomla, or Plone. Think of MT or WP as a specialized, $20 Lego car model kit. It's meant to build that car and maybe with a few variations. And perhaps you can squeeze something else out of it. But if you are looking to do more than build that kind of Lego car, you really need the mega tub of Legos or maybe K'nex (depending on the range of things you want to build).
This is a good analogy, too, because many newbies to CMS's are easily frustrated because they can't find directions to build exactly what they want. Documentation on the CMS websites is much like the booklet that comes with that giant box of Legos or K'nex. It'll have a few examples, but the software designers just can't explain how to build the whole range of websites and their variations (on the other hand, it is very easy to do so with WP or MT). For that, one has to become familiar with the building blocks, learn to use one's imagination, and look at what others are building with their box of blocks.
So the best advice is to dive into a CMS and begin experimenting. I don't know about anyone else, but the way I learned to build with Legos was just to sit down and play with them. And like playing with those Legos, there were many times when I have had to tear down a site and begin again until I really got good at working with the pieces.