When creating a mapping application, it's extremely important to stay away from that. There's no point to creating an application that's just a styled Google map. There has to be a point to having your application centered around a map. I think a drawback of using Google maps (or any of the mapping services) is that it's so familiar to people. A lot of people might assume that the application your building is just a Google growth. Fortunately, Google has a good deal of features built into the API to keep that from happening. You can pretty much design what your map will look like down to the most seemingly superfluous of details.
Mapping applications are so awesome because of how interactive and driven by visual exploration they are. I think there will be a lot more of them doing things that we don't expect to see in the future.
One last note: Any TileMill developers out there? What would your advice be for someone who is making a mapping application/considering moving over from gmap?