In addition to some very cool features (an incredibly intuitive mobile interface, leveraging Flickr for both the social network and a place to park my geopresence, etc.) I love the support for "private maps". This allows for pinpointing my location within a venue. It rocks IMHO. Great job Chad, Karon, Sam, Ed, Jonathan, etc.Chad does a great job describing how this work evolved.
And Edward gives yet more detail.