KDE 4.6 will be slightly more ambitious than the last couple of releases. Judging from the recently-announced first beta, it will include the usual behind-the-scene enhancements, and additions of interest to developers. But it will also include some concrete improvements to the desktop, especially with the changes to the Dolphin file manager and the so-called re-introduction of Activities.
You can download the source code for the beta from the usual mirrors. However, the beta, officially numbered 4.5.80, has already been packaged by many distributions, including Kubuntu (Ubuntu's KDE distro), openSUSE, and Fedora.
The Kubuntu packages were prone to crashes when I tested, but that will probably change by the time you try them. By contrast, the live image for Rawhide, Fedora's development repository, was slow to load but stable.
Many of the changes in the beta are of interest mainly to developers and not directly observable to casual users, although the changes may affect development in future releases.
Some of these changes are designed to extend the use of KDE to mobile devices. The beta supports QML, a language for developing interfaces for mobile devices. Similarly, Solid, KDE's engine for hardware interaction, is moving away from reliance on HAL, the Hardware Abstraction originally developed for workstation desktops and adds support for such replacement technologies as upower and udev.
KDE activities panel
The beta also supports the use of device targets, and makes PowerDevil, KDE's power management system more customizable. However, the benefits of these changes are unlikely to be realized in the 4.6 release, and many will probably go largely unnoticed by average users when they do come into play.
Only slightly more noticeable is the completed deployment of Akonadi, KDE's personal information management (PIM) engine, which in previous releases was confined to supporting a few applications like KMail and KAddressbook. Since this change unifies PIM storage in a single database, in theory it should increase the efficiency with which PIM applications interact, but the greatest benefit will probably be to developers working on future releases.
One improvement on the desktop not emphasized in the release notes is the continued growth of widgets for the desktop or panel. When the KDE series was first introduced, the selection of widgets was sparse, but their number has increased with each release, and the 4.6 beta appears no exception.
The available widgets varies with the distribution, so distinguishing those just be introduced from those just added by the distribution can be difficult. However, if I am correct, the beta introduces a number of monitors for hardware, a Facebook widget, and a keyboard locale switcher -- all welcome additions to the growing ecosystem of mini-applications.