The first change it jumps into the eye is the new Plasma appearance, called Air now. It is brighter than the old one, but I found it looking not so appealing, at least not for the desktop widgets. Here's how the desktop looks in Kubuntu Jaunty, with several widgets running on the desktop:
New look for Plasma
Dolphin, the default file manager
KRunner received several great improvements: it now displays the applications to be run as a list (see screenshot) and this list can be customised too. See below:
According to the announcement on the KDE official website, this release got many Plasma improvements, several game updates, PolicyKit integration, and several other changes or improvements.
For those who like desktop effects a lot and have a video card capable of supporting OpenGL, a combination of wobbly windows + window transparency and the magic lamp minimize effect should be very satisfying.
I noticed plasma-desktop eats constantly 30% CPU (Core 2 Duo 1.8GHz, 1 GB DDRAM2 and nVIDIA 7600GS), which is a lot greater than in KDE 4.2 which comes by default with Kubuntu. Still, this is just a release candidate, and the upgrade was performed over an existing KDE installation using dist-upgrade in Kubuntu.
Konqueror in 4.3RC1 - KDE web browser
Okular, the default document viewer in KDE4
Gwenview is now included in the KDE distribution