New to Google Earth 5.0 and later:

• Support for displaying underwater terrain, water surface and underwater 3D models.
• Support for historical imagery including new UI element to allow selection of historical imagery assets.
• New touring feature that allows a tour or presentation to be recorded and played back. It also allows sound recording and playback.
• Support for full HTML, Javascript and iframes in balloons shown in Earth.
• Support for Mars imagery/terrain/layers through drop-down menu for sky and planets.
• Support for 15 new languages, for a total of 41 total languages.
• New unified user interface for time in Earth, supporting sunlight, time-based KML, and historical imagery.
• Redesigned user interface icons and splash screen.
• Support for opening description balloons by clicking on lines and polygons.
• Support for arbitrary corner positions for ground overlays
• 3D buildings enabled by default on capable machines.
• Full scene anti-aliaising in OpenGL mode on Windows systems.
• GPS device track import (46 new devices) and real-time tracking now in free version. You can find more information at http://earth.google.com/userguide/v4/ug_gps.html
• More prominent bouncing of labels for KML content.
• Separate installers targetted for installing for All users or single user on Windows.

Improvements in Google Earth 5.0:

• Faster start-up time.
• Earth honors cache settings more rigorously.
• Faster processing of KML files.
• Significant performance improvements for 3D buildings in DirectX mode.
• Enhanced support for anisotropic filtering.
• Improved level of detail computation for better performance and quality.
• Use OGC standard KML name when saving KML files.
• Support for installers for users with non-administrative priviledges on Vista.
• Improved performace for regionated KML files.
• Improved performance in Pro version printing images, saving images and capturing movies.
• Added HD view / output sizes.

Known Issues in Google Earth 5.0:

• In low memory situations, 3D buildings may not get displayed on Earth. Some of the workarounds include increasing cache size from options menu, restoring defaults from options menu, restarting application and clearing memory cache.
• On some windows systems, sound recording may not work. In this case, you can fix it by installing Microsoft Visual C++ 2005 Redistributable Package (x86).
• For KML content, balloon formatting may be different as compared to previous versions and an unnecessary horizontal scroll bar may show up.
• Switching to sky from Mars does not change the search box to do sky search. As a workaround, you would need to switch to Earth and switch back to sky.
• Sometimes, overlays drawn on ocean surface may also get drawn on the underwater terrain.
• Missing images in balloons may cause balloons to flicker.
• In KMZ files with network links, styles from a link do not always override styles from parent file.
• KML files saved from client have unnecessary extra namespace declaration.
• Using a "fly to" (by double clicking a place mark) when recording a tour might lead into a jitters in recording.