Update: Thursday morning I upgraded our network to WordPress 4.0. Everything appears to be running smoothly.
After testing WordPress 4.0 for about a week, the new version looks ready to deploy to our network. Normally we would wait a bit longer; however, we try to avoid feature updates to WordPress core during the semester. That would mean having to wait until late January, and WordPress 4.1 should be out by then.
The jump from version 3.9 to 4.0 is no more significant than the jump from 3.8. They just chose not to use “3.10” as their new number as Drupal or OS X would do. There are three new features that will impact content creators on our network directly.
Media Library Grid. There is a new grid view in the Media library, with the Edit dialog displaying in an overly, rather than a new page. The list view is still there and functions the same as in the previous version.
Seamless Media Embeds. Embeds now preview right in the visual editor. This includes image galleries and media that uses oEmbed (YouTube, Vimeo, Media Central, SlideShare, Twitter). For example, as soon as you paste a YouTube link into the visual editor, a progress bar appears, and a preview of the video is immediately visible. You can even play the video right inside the visual editor. If you have a large number of video embeds in a post, this might slow down the initial rendering when you go to edit a post. There is one huge annoyance with this feature. After the live preview of the embed is rendered, WordPress inserts the cursor before the embed. I would expect the cursor to be inserted after.
Intuitive Editing. The last change auto-expands the visual editor as you type and then keeps the toolbar in a fixed location as you scroll back upward. This feature takes some getting used to, but it does make the visual editor seem more productive. Content authors who dislike this feature can turn it off individually, via the “Screen Options” button at the top of the Add or Edit pages, as depicted in the following screenshot.