Main changes in this release are as follow:
Qt Quick in Plasma Workspaces : Qt quick is making it’s way to KDE. New version of Plasma workspace, Plasma Quick is written on top of Qt Quick which enables deep integration to system and resources. Various plasmoids like the system tray, pager, notifications, lock & logout, weather and weather station, comic strip and calculator are rewritten in Qt Quick.
New locking mechanism and screensavers : New locking mechanism are now written in Qt quick, which provides more flexible solution to secure plasma workspace. New Qt quick based screensaver engine will provide a new and easy way to create animated screensavers.
Notifications : Now notifications are being rendered by Qt quick. This cleans up the notifications specially like power management.
Apart from these changes related to Qt quick, There are many improvements in KDE applications and system performance. In case of applications notable changes are :
Improved zooming in Okular : Using new technique called tiled rendering, zooming in Okular is improved. In this method only visible parts of the documents are being rendered. This reduces the memory consumption.
Kate is now extensible using python plugins and also many other improvements are made regarding user feedback. Ktouch, A utility to learn touch typing has been rewritten from scratch to improve user interface.
System related improvements includes color correction, Faster indexing and new print manager.
Color correction : Gwenview, KDE’s default Image viewer and Plasma window manager now supports color correction to allow more natural representation of photos and graphics, Using different color profiles for different monitors.
Faster indexing : In this version KDE uses new indexer which allows the Nepomuk semantic engine faster indexing of files. The new Tags kioslave allows users to browse their files by tags in any KDE-powered application. We have already seen this feature in Amarok 2.7 beta which was released recently.
In this release some games like Ksudoku and KJumpingCube are now improved as many parts of libkdegames are rewritten.
However list is not yet over, There are many changes in this version, you can found details here. There are some distributions which will provide upgrade to this RC through their 'unstable' channels. If you want you can grab the source code and compile it yourself.