Wordpress, Joomla, and Drupal pretty much go in that order for ease of use, what they're best for, and how powerful they are.
Wordpress is the easiest to use, most intuitive, least powerful, and best choice for simple sites the likes of which you'll typically find around a place like this (SEO/marketing oriented forum).
Drupal is the most difficult to use, least intuitive, but also the most powerful if you're a programmer.
Joomla falls in the middle.
If you're asking this question, I'd say there's a 95% chance that what you want to do is simple enough that you're best sticking with wordpress. If you use joomla for something simple, you're just adding a lot of unnecessary complexity for a less optimized, less intuitive version of the same thing. Also, Joomla has (by far) the highest percentage of commercial plugins of the three, whereas with the other two most plugins are free.
If you're looking to build a community site, like a social networking site or something that's ripe with user created content then yea, joomla is a MUCH better choice than wordpress. But if you're just looking to build a site that you can put some ads on or sell a digital product on or something like that, you'd be foolish to use joomla or drupal over wordpress.
Of my ~20 sites that use a CMS 1 is joomla, 1 is drupal, and the rest are all wordpress.
Wordpress is much better suited for sites that you want to get up quickly and spend the majority of your time doing SEO on (hence its popularity here). Joomla/Drupal are better suited for the kinds of sites that you're going to be spending the next several months developing before you go live with them.
I'm actually working on a detailed article comparing the three, and what their best uses are.