Responsive, platform/screen agnostic back-end for the win.
Not enough value contrast on the back-end styles for my tastes. A bit more value differentiation between the darkest gray (#222) and the slightly lighter dark gray (#333) would have been nice (say a #444). The other styles to choose from are interesting. Some are quite vibrant but again suffer from not enough value differentiation and some come across as overly monochromatic.
Larger font sizes – good.
The Dashboard is nice and roomy, removing some of the previous “clutter”. I still think the At A Glance should be a bit more prominent. A Dashboard is supposed to provide a useful, timely information about your WordPress installation.
Appearance is quite nice – it’s very clear about what theme is active, how many themes you have, and, a good way to help users add more themes.
Widgets and Menus are a pain-point and will need to be looked at in the future. With more complex sites these can be quite burdensome to use. They just don’t scale up all that well.
Plugin management – well done. Good indicators on which plugins are activated and which ones need updating.
Final thought: another good release, primarily backend user interface and usability focused, laying the visual groundwork for future developments.