I think by far the best part of Sitefinity is how it is built on and makes full use of the ASP.NET architecture. Because of things like Code-Behind, Themes, Skins, and other foundations of the Framework, Sitefinity is more open and extensible than any other CMS platform I've ever used.
A direct consequence of the separation of content vs design is how easy it is to "swtich themes". It's as simple as populating a new Theme folder with new css and images mapped to the css classes defined in your page. It was a simple matter to take this one ...