What took a little while to click with me, and really hit it home for me once it did, is how simple ProcessWire's approach to content and data actually is. Its primary admin UI looks like it's for editing a straightforward site hierarchy, and it certainly can be, but the key is that its "pages" don't have to be web pages. A combination of some simple PHP and some quick setup in the admin lets you build all sorts of things.

At its core, ProcessWire just gives you an extensible, hierarchical (and relational when you need it) model to build upon, and then gives you a really slick jQuery-style syntax by which to access it. You build templates in PHP, but the selector syntax is so easy to work with, it brings to mind WordPress theming.

One of its founding principles is that it is markup-agnostic. I'm using it right now for an XML source that feeds a site (and possibly later a mobile site, an app, or anything), with the XML being a ProcessWire template.