Google Maps strength is largely due in part to their surveying of roads with those funky looking vehicles I think most of us have seen. If one were to develop an app that could use anyone's GPS to do the same as what Google is paying money for... it could pose a real threat to them. A "crowdsourced" map of sorts..
I think their navigation offering is another strong moat in terms of just plain old maps.
I'm sure Google is mining all of that sweet, sweet GPS data from people using their navigation. I can imagine some machine learning of aeriel imagery and some sort of GPS path clustering helps them get a lot of the last mile.
Navigation also helps them get real time traffic data and allows them to learn how costly certain road segments are.
