The Plasma Workspaces gain from extensive work to KDE’s compositing window manager, KWin, and from the leveraging of new Qt technologies such as Qt Quick. For full details, read the Plasma Desktop and Plasma Netbook 4.7 release announcement.
Many KDE applications are updated. In particular, KDE's groupware solution, Kontact, rejoins the main KDE release cycle with all major components ported to Akonadi. The Digikam Software Collection, KDE's feature-rich photo management tools comes with a major new version. For full details, read the KDE Applications 4.7 release announcement.
A wide range of KDE and third party software will be able to take advantage of extensive work in Phonon and major improvements to the semantic desktop components, with enriched APIs and improved stability. The new KDE Telepathy framework offers integration of instant messaging directly into workspaces and applications. Performance and stability improvements in nearly all components lead to a smoother user experience and a reduced footprint of applications using the KDE Platform 4.7. For full details, read the KDE Platform 4.7 release announcement.
So much more information than you can shake a stick at on the original KDE site.