I upgraded to Marshmallow when it first came out and on the surface there are only a few minor new features.
However I just discovered one great new feature for Nexus users and that is direct support for OTG.
If you are wondering what OTG is and why you would want it well it stands for On The Go. On The Go basically enables you to put a USB stick into your phones USB port and transfer data but it's a bit more than that as this Wikipedia page explains: https://en.wikipedia.org/wiki/USB_On-The-Go
Previously, for some bizarre reason, if you wanted to use OTG on a Nexus device you had to root your device and install an app such as Stick Mount. Many Samsung and other phones support OTG already right out of the box.
Well Google have now seen fit to give us OTG as a built in feature, no need to root and you don't even need to install a third party file explorer to see your files either, you can do it straight from the device. Even so you might still want the extra features that apps such as ES File Explorer offer.
For those of you with the newer USB type C ports you'll need to buy a type C OTG cable or a type C USB drive. Yet another "Universal" standard.
OTG does of course work on various unrooted pre-marshmallow nexus devices - you can use it for all sorts of handy things apart from mass storage - but the native USB drive read write is a handy improvement - I must give it a try!