Google Earth 4.3 will feature a bunch of interesting new features. The new edition finally incorporates Street View but there are also other improvements like day/night lighting effects, new and faster 3D models, improved navigation and imagery dates.
First off, we have new day/night lighting effects. You can turn on a real-time sunlight which shines on the new atmospheric lighting Google has added. Look for the new sun icon in the upper middle (next to the new Sky button). Once it loads, you will see the current position of the sun. You can grab the time slider to change the sun over the course of a whole day so you can see the sun from sunrise to sunrise. Note that the lighting effects the atmosphere, so you actually get color changes near the horizon, and on clouds (if you turn on the Weather->Clouds layer)
A full overview can be read at Google Earth Blog and here's a YouTube video from Google that illustrates the new features: