Sidebar and new notification menu
TL;DR
The new sidebar lets you customize your navigation so you can get to your favorite parts of your community, and the new notifications menu that lets you more easily decide which of your notifications deserve your attention now and which ones can be dismissed or be saved for later."
The new notifications menu has tabs for each type of notification so when you’re looking for those two recent replies but have gotten a few more recent likes and mentions, you can find them easily without navigating away from the current page.
More details here for those that care.