Ever since Android added support for native screenshots way back in Ice Cream Sandwich, there's been a handy notification after the screenshot has been saved.  In the new M preview the share button on that notification is joined by something else—a delete button.  Fantastic. 

The delete button is only visible in the expanded notification, just like the share button on Lollipop and earlier.  Tap it and the notification goes away.  The file will be cleared from your internal storage, but not from any cloud container it might have synced to in the meantime. 

This is something that's been in at least one custom ROM for a while (CM), so it's not entirely new.  Also... I feel like a confirmation dialog might be warranted here, but it's still a cool feature.