I've never seen anything as easy to use as Serendipity. And the support is just awe-inspiring. The installation is simple, the plugin library is extensive, themes are easy to create, everything about it is immensely satisfying.
But the single greatest factor in the success of any project is its community. And Serendipity delivers! The forums are active, and the founder often answers questions directly -- usually the same day. Everyone is willing to help out, and there's nothing wrong with being a newbie. The developers are there to help out, too. The program is easy for other developers to extend, too, with a plugin framework that's clean and simple. No wonder there are so many plugins.
You'd think that many plugins would be hard to manage, but of course that's a problem just waiting to be solved with a plugin. Sure enough, every installation comes with SPARTACUS, a plugin that monitors the main Serendipity site for updates and new plugins. You don't have to download, unzip, and FTP any more. You just start SPARTACUS and pick the plugin you want from a list.
With other blogs, I was exasperated with the poor response time for support, or frustrated at bad decisions, leading to slow, insecure blogs. With Serendipity, I've found paradise. I wanted a fast, simple blog, and I found inner peace. Now that's Serendipity.