KDE has released the release candidate of the new versions of Applications and Development Platform

This release does not include Plasma Workspaces since it was frozen for new features in 4.11.x. The Development Platform has been virtually frozen for a number of releases, so this release is mainly about improving and polishing Applications.

A non complete list of improvements can be found in the 4.12 Feature Plan
http://techbase.kde.org/Schedules/KDE4/4.12_Feature_Plan

With the large number of changes, the 4.12 releases need a thorough testing in order to maintain and improve the quality and user experience. Actual users are critical to maintaining high KDE quality, because developers simply cannot test every possible configuration. We're counting on you to help find bugs early so they can be squashed before the final release. Please consider joining the 4.12 team by installing the release candidate and reporting any bugs https://bugs.kde.org/

  KDE 4.12 Release Candidate