I have been working with the PowerPoint 2016 preview for a while now, and overall my feedback is very positive (see my PowerPoint 2016 review here).
There is one small thing that keeps me going back to PowerPoint 2011 though: the ability to customise the toolbar at the top of the screen. My set up has not really changed since this blog post from 2008. When working in PowerPoint I constantly need to access buttons that align/distribute/crop/flip and send objects to the back (and the drop shadow button to kill drop shadows). With my custom toolbar, I basically circumvented the majority of the PowerPoint user interface and created my own.
Hopefully Microsoft will include this feature in the final release of PowerPoint 2016.
Image credit: Kate on Flickr