Plasma active device profiles documentation
The thing we always stressed out in Plasma Active is how the system is designed to fit a whole device spectrum, even if the first two releases are explicitly about tablet devices (and in the near future this is not likely to change ;)
Plasma QML documentation
Brand new Widgets explorer
The rewamp of the KDE Plasma Desktop is continuing: in the 4.8 release it got a completely new QML based device notifier (already quite improved for 4.9), now a new redesign has landed in master, scheduled for the 4.9 release of KDE Plasma Desktop.