PlayingWithWire has a usability comparison between WordPress and Joomla! that has gotten a lot of attention on Slashdot and the Joomla! forums. The debate as is as you would expect: WordPress is more usable; Joomla! is more powerful. I don't disagree with either position.
I think this highlights the fact that WordPress is at a point in its progression where it can handle many simple web content management use cases but has not yet achieved a level of complexity as to detract from its usability. It has truly become a viable lightweight CMS - not just a blogging tool. This makes WordPress and platforms like it (Movable Type, Expression Engine, etc.) disruptive technologies in the classic Christensen disruption model where a simple technology reaches a point where it can compete against a more complex incumbent that over-delivers in functionality.
The disruption model usually focuses on price; the challenger technology is cheaper than the incumbent. Not that long ago, we were talking about open source as the disruptor (we still are. Just look at all the chatter about the Linux powered Netbooks). In this case, however, both are open source and free. Here, the cost is in the effort it takes to understand and use the technology. Writing good content is hard enough without the barrier of a hard to use web content management system.
There is a clear trend of companies leaning toward simpler technologies that may not meet all of the extended requirements but are very effective in the primary use cases (in this case, publishing pages and articles). There comes a point where a simple tool reaches its limitations but many companies are prepared to make compromises or doubt whether they will ever need the fuller feature set - at least in the near term.
The incumbent CMS products are not taking this lying down. Many of the commercial products on the market offer a simplified, "task-based" user interface for the casual user as an alternative to their traditional "power user" interfaces. But even unused and unseen functionality has a cost in the complexity of implementation and the cost to support. If your website is very simple and you don't have any power users on staff, a simplistic, lightweight CMS (like WordPress) may be sufficient.