There are a few noteworthy changes to the editor in WordPress 4.7.
Some of the toolbar buttons have been rearranged to make them easier to access and to encourage proper use of the HTML elements they insert. The headings drop down is now moved to the top row, and the strike-through and horizontal rule button are moved down. This also reflects their usage.
The underline and justify buttons have been removed from the bottom row. Underlining is a bad practice as readers can confuse it with links (bad accessibility), and it does not insert a semantic element. Justifying has uneven browser implementation, and in many cases is bad for readability. Keyboard shortcuts for both will keep working.
For more information see #27159 that also has links to the discussions in Slack.
Labels for keyboard shortcuts have been added to the tooltips for buttons and inside drop downs to make them easier to discover.
As always, feedback is welcome.
Ella and Andrew