I'm not a WordPress expert, far from it, though I've helped out on a couple of sites that use it. However, it seems a very workable CMS for low-end sites. Some people of course use it successfully for somewhat larger ones. Drupal and Joomla, whilst I know less about them, are more suited than WordPress to medium and large sites. Drupal in particular is used for some very large (and very active) sites indeed. I've made more use of Percussion CMS (some time ago), Sitecore and now Kentico.
CMSs are, I think, underestimated in the comments above. If you're building a decent-size site, and want consistency and the ability to write pages without knowing HTML in detail, they are the answer. The big issue is that people forget that they are for content management (the clue's in the name folks), not content creating and forgetting. If you just use them to pile in more stuff in a random fashion, you'll get a randomly-arranged site.