Multisite, now faster than ever!
Cached and comprehensive site queries improve your network admin experience. The addition of WP_Site_Query and WP_Network_Query help craft advanced queries with less effort.

Theme add and activation work flow changes to one screen

Editor improvements – shows broken link

Lost Internet connection – drafts saved locally to browser

WordPress Dashboard uses native fonts from Operating System resulting in loading improvements (speed up)

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JavaScript Library Updates
Masonry 3.3.2, imagesLoaded 3.2.0, MediaElement.js 2.22.0, TinyMCE 4.4.1, and Backbone.js 1.3.3 are bundled.

This release was led by Dominik Schilling with the help of 272 Contributors.  Read the official release page for WordPress 4.6