Keith Fraley says:

I really think there are two notable points with this release, that aren’t prehaps getting that much attention in the blogosphere

You know what, he’s got a great point. Subsurface navigation is a huge deal because that is one of the biggest complaints I’ve heard about Google Earth from some clients. Of course keep in mind that ArcGIS Explorer doesn’t have anything subsurface to look at by default so you’ll need to get some data layers to play with to actually see something other than blue once you go underground.