There are very significant differences between this release and all 1.x releases. 1.x releases followed most of the conventions established by Seven and merely changed the styling of the CSS attributes. This version changes the look and feel of the design significantly to follow more current trends towards flat design and to conform to the style guide of all Acquia maintained projects.
Additionally this release uses several new techniques including Sass, compass, and svg icons to provide a more modern flexible framework that can be more easily extended.
Note: A central goal of the Ember project is to create module CSS overrides for all of the modules included in the DF project so that outlier styling which gives DF an uneven and disconnected experience are normalized.