5.1

new binary kwin_wayland
kwin_wayland supports starting the X-Server it needs to connect to
Wayland (client) connection is hold in a thread
use of libepoxy

5.0

Code base uses C++11
Port to Qt 5 and KDE Frameworks 5
Support for "native" graphics system in Qt got removed
DBus interface got cleaned up, deprecated methods are removed
New Window Switcher mode which puts thumbnails into a sidebar
New Background Contrast Effect
Lanczos Filter does no longer check the graphics effects level
Libkdecoration received cleanup and various new API features
kwin_gles does not exist any more, KWin follows Qt's OpenGL choice
Window Decoration configuration module can render interactive previews for QWidget based decorations (e.g. Oxygen)
Window Decoration (and Alt+F3 menu) can follow the color scheme from the decorated window
Window Specific Rule to specify the color scheme for the window decoration of a given window
KWin effects specify an URL to a demo video (used by new effect configuration module)
Client uses QIcon instead of set of QPixmap for the window's icon
Better support for windows of type notification
Built-in effects are no longer provided as a plugin, but linked against KWin
KWin is kept responsive while the effects are getting loaded
Wayland backend for the XRender scene
QPainter based compositor for Wayland
Better detection on when KWin should use Wayland
Compositing gets enforced in Wayland mode
Support for Client showing on ScreenEdge, see KWin/Screen_Edges
KDecorations installs a KDecorationsConfig.cmake
Removed build option KWIN_MOBILE_EFFECTS
Removed build option KWIN_BUILD_SCRIPTING
on all desktop window decoration button is only shown if at least two virtual desktops
Removed taskbar thumbnails effect
Removed OpenGL 1 (legacy) support (including build option KWIN_BUILD_OPENGL_1_COMPOSITING)
Outline uses QtQuick
Config option to switch between GLX and EGL
Only one DBus service: "org.kde.KWin"
DBus object /TabBox is no longer exported
TabBox does no longer support the embedded mode
Binary renamed to kwin_x11
kwincompositing KCM got replaced by a new kwineffects kcm which just configures all the effects and by a new kwincompositing kcm which just configures the compositor.