Google Earth 6 doesn't have too many new features. You can now use Street View just like in Google Maps by dragging the pegman icon. "To view street-level imagery for a specific location, zoom into an area at an altitude of approximately 500km. You will see a pegman icon appear at the top right below the navigation controls. Click and drag the icon across the 3D viewer. A blue border will appear around roads that have street-level imagery available," explains Google.
Another new feature is the "3D Trees" layer that can be enabled from the "3D Buildings" section. Google Earth includes 3D models for city parks (San Francisco, Chicago, Tokio, Athens) and remote forests (Amazon Forest).
Historical imagery is now more accessible: just click on the date of the oldest imagery in the status bar and you'll be able to see all the historical images that are available for the same location.
Google Earth 6 also adds ground-level navigation that lets you explore 3D buildings and 3D trees, 3D measurements for heights and widths of buildings and a much better Tour Recorder that improves motion fidelity.