This article comparing the usability of Joomla vs. WordPress has already been linked by everybody’s uncle, but it’s still worth a look.
I find it amusing, however, that none of the comments so far on that blog post mention the commitment that the core WordPress team appears to have on making blogging fun. If you start with the goal of making something fun, then add sophistication to make it flexible without being complex, you’ll get a very different result than you would if you started with different goals.