Among the improvements that Seigo talks about in this Plasma Workspaces 4.8 posting is the OpenGL ES and Compositing Performance improvements (thanks to Martin Gräßlin's continued work on KWin), lots of bug fixes, and improved power management. The improved power management is also fixing a large number of stability/predictability-related bugs, such as for handling multi-screen power management situations, etc.
Aaron's blog post can be read here. Related reading is the performance overhaul of KDE KWin and Plasma Active slims down KWin. KDE also has initial plans foe Wayland in KDE SC 4.8, but that wasn't talked about in this blog post.
Last week Aaron blogged about other Plasma Workspaces improvements. The previously talked about items included screen locking, Qt Quick splash screens, input method panel, on-screen keyboard, picture of the day support, Plasma QtComponents, an improved window switcher, and many other KDE desktop items.
The final release of KDE Software Compilation 4.8 is expected on the 25th of Janaury. The release schedule with development builds and other details can be found on this KDE TechBase page.