The new features added to this are
Oceans - Earlier versions of Google Earth of course had oceans but not clear enough. With the new Google earth 5.0, nooks and crannies on the surface of the earth inside the oceans can be viewed. That makes it so good. Now you can dive into the ocean and check out some deeps!
History Imagery - The earlier versions of Google Earth showed pictures of locations which were taken sometime back. Now, Google Earth can take you back in history and show you how the locations looked like.
Recording Tour - Sharing your tour route has always been troublesome while explaining it to others. But not anymore with your Google Earth 5.0. You can now record your tour with a click of button. Click on the record button and fly through the destinations you went by. Click stop and then you can save the tour route. Upload it to YouTube and share it with the world. Simple! Isn't it?
3D Mars - Now this is my favourite. Google Earth had already released the sky view in its previous versions. The clarity was just quite lousy. Now, Google in collaboration with 'NASA' has released this new feature - 3D Mars. By selecting Mars from the Toolbar, the surface of the Mars can be viewed with clarity and resolution high enough. You can see Mons Olympus, the biggest Volcano in the solar system. All those things you read in General Knowledge books can be experienced now. Isn't that Cool?
Check out this video released by Google through its blog.