Start Menu. It appears File Explorer and All Apps have new wireframe icons.
Also take note of the Music tile - it has a new icon. The Video app is now called Movies & TV which matches the page in the Windows Store (Beta).
That makes a lot of sense actually as you now buy your Movies & TV and them play your Movies & TV in an app with the same name.
The Mail and Calendar tiles also have new icons that are similar to the new ones in Windows 10 Mobile Build 10136.