Nevertheless, this is quite a change with how it worked for previous versions of Office as they only got (bar a handful small exceptions) security updates and bug-fixes.
Any real changes had to wait for the next version release.
Office 2016 will get the same treatment so future updates can introduce new features as well.
When you switch your tablet into portrait mode, Outlook will show the message at almost full width of your device to optimize the reading experience.
The Folder Pane will be shown minimized with all your Favorite Folders at the left side of your screen and with a Back button at the top to switch to the Message List.
When the Message List is shown, a icon is shown to expand the Folder Pane and show all your folders (similar to the way you can minimize the Folder Pane in Outlook 2013).
For more information see: To-Do Bar Calendar functionality in Outlook 2013Even though Outlook 2016 looks very much like Outlook 2013, is does include quite a few improvements.
However, not all of them are visible changes but mainly technical (like performance improvements and the reduction of using foreground threads to perform certain operations) or are not visible to everyone as they require you to use an Exchange or Office 365 account.
Outlook 2016 also still offers support for the the RPC over HTTP Exchange connectivity support for backwards compatibility with Exchange 2010.