Office Insiders who enabled “Coming Soon” can experience the new Simplified Ribbon. This is a one row toolbar, with the most used commands visible, with other commands in an overflow menu. (It looks a lot like the toolbars we had before the ribbon. Yes, what is old is new again.)
With the simplified ribbon, if you like to hide the ribbon can now have some buttons visible, without wasting a lot of screen space. If you prefer to keep the ribbon open, you can recover some of the screen space the full ribbon wastes. If you need to access the full ribbon, it’s one click away, using the V on the far right of the ribbon.
I like the simplified ribbon - a lot. My only complaint is that it’s a separate ribbon and my customizations on the classic ribbon aren't on the simplified ribbon and I need to customize both ribbon to avoid the extra step to show the classic ribbon and find my customized commands.
The folder list is a mess, in my opinion. While I like having the shared mailboxes, public folders, and archive mailbox grouped with the owner mailbox, the calendar navigation pane is a mess because the mailbox name was removed from the folders.
I don’t like some of the other changes, including a new compose message header. The font in the To/CC/BCC fields appear to be a little larger but the header area is huge and wastes a lot of space on smaller screens; the space you save with the simplificed toolbar is taken up by a larger header area. (I'm hoping a future update makes it a bit more compact.)
Other new features or changes include the simplified account settings dialog. For more information on this, see Change your Password and Server Settings in Outlook 2016/