There are numerous improvements implemented in this version, most of them for the next-generation Wayland display server, which we hope to become the norm for the most prominent distros later this year, as well as various other goodies and countless bug fixes.
For example, the first thing you’ll notice after upgrading to KDE Plasma 5.7 is the all-new, unified login screen. The tray area has been revamped a little, there’s a new task manager with improved performance, the Oxygen and Air Plasma themes have received various enhancements, and so where the Breeze icons, which now features a tint to match the highlight color scheme, and the Jump List Actions feature was extended to be accessible from KRunner.