KDE 4.3.0 is out, and it is a great release. It is unlikely that any one specific thing will strike the user as the most noticeable improvement; rather, the overall user experience of KDE has improved greatly in KDE 4.3.0.
The release’s codename, Caizen, is a Japanese philosophy that focuses on continuous improvement throughout all aspects of life. That has been the goal of the KDE team for 4.3.0: polish, polish, polish. The statistics from the bug tracker speak for themselves: 10,000 bugs have been fixed. In addition, close to 63,000 changes were checked in by a little under 700 contributors. That is not to say that the KDE team did not add a large number of new features: 2000 feature requests were implemented in the past 6 months, meaning that any user’s pet feature might well be among the improvements KDE 4.3.0 brings.