The HiFi theme editor is probably of most interest to developers. All of the templates, styles, scripts, images and fonts that are associated with a site can be bundled into a theme. These can be edited directly from the browser!
The tabbed text editor is based on Mozilla Skywriter, an open-source project based on HTML5 that shows great promise and is a pleasure to use. To the left of the editor is a listing of your theme's files. To the right is an embedded version of the HiFi documentation. This makes it really easy to find what you're looking for when working on a site.
The HiFi Theming system is discussed at length in this documentation. We believe it is a true step forward in both simplicity and power and are very proud of it.