Relative Openness of Mapping APIs

I’m interested in drilling down on the question of why Google Maps has become the platform of choice for location mashups, rather than Yahoo! or Microsoft or MapQuest. Was it just being in the right place at the right time, and getting the momentum, or is there something about the API or the terms of service? There are hundreds of Google Maps mashups versus tens of mashups for Yahoo! Maps or Microsoft Virtual Earth, and only one for MapQuest.

I asked Schuyler Erle and Rich Gibson, co-authors of Mapping Hacks and Google Maps Hacks this same question yesterday. Schuyler replied with some thoughts, which he’s since blogged on mappinghacks.com. But I’d love to know more. Why are you guys continuing to choose Google maps? What features, if any, would competitors have to add, to get you to switch?