The release announcement points to improved support for mobile device, visual refinements that go well with the Plasma Workspaces, improved performance of many KDE Applications and more APIs to Qt Quick among others.
Diving deeper in the improvement aspect, “several components of the Plasma Workspaces have been ported to the Qt Quick/QML framework” in KDE 4.10. The APIs will allow developers to contribute to KDE through the use of Plasma SDK. Developers can write Plasma widgets and widget collections using the the Qt Markup Language (QML).
You can find more information about the new release and the changes at KDE.org.