The Plasma desktop shell comes with a new default theme called Air, which looks a lot less heavy than the previous Oxygen theme. Plasma has also been improved performance-wise, and it takes up less memory. The folder view widget now has a feature which allows you to drill down into folders without having to open Dolphin.
The SystemSettings tool can now employ the tree view again (KDE 3 people, rejoice!), and it also has been improved performance-wise. Dolphin received some love as well, such as small previews of files within a folder, and its configuration panel has been improved.
This is just a selection. KDE 4.3 comes packed with changes and improvements, so have a look around the release announcement to see if there's anything of your fancy. Distributions will pick this new release up soon enough.