Today, the first beta of WordPress 3.9 was released with plenty of tweaks and new features. While not quite as noticable as 3.8, there are numerous changes for both developers as well as end-users.
We encourage you to deploy a development site to play around with it, especially if you use or develop a plugin that modifies TinyMCE. For a full list of all changes, take a look at the WordPress release news posted today.
The update to TinyMCE has introduced better handling of pasted content, as well as much cleaner HTML markup. Pasting some formatted content from other sources will no longer cause unexpected results within your post/page entry field.
Widgets on live previews
Widgets can now be added and re-arranged from the live preview screens when using the theme customization options. This should allow much greater usability for novice individuals to place widgets and see what they should look like easily.
Theme installer redesign
The theme installer has now been redesigned to provide the same experience as the theme browser. Installable themes are now much easier to navigate through.
Gallery live preview
Galleries can now be previewed live from within the editor. This makes it much easier to see that you have in place without the need to open the page that the gallery appears in.
Drag and drop images
Images can now be dragged and dropped into a post or page using the editor. No longer will you need to browse for the image before uploading it.
Better image editing controls
Images inserted into the editor can be cropped and rotated much easier, and without opening up a secondary window to do so.
MySQLi is now used
If your WordPress site is running on a recent version of PHP, MySQLi will be used to better the performance of database functions.
Autosave integrated with the heartbeat
The autosave functionality was moved into the standard WordPress heartbeat.
As TinyMCE 4.0 intorduces major changes, developers may need to modify their code if they are extending upon it.