2011 was a year of intense change in the mobile development world. A few years ago, decisions were easy: build for iPhone. Now, there are not just phones but tablets. Android has three major versions in the wild, the most prominent by far being the 2.X line, which is considered ancient. RIM’s BlackBerry is struggling to land new customers and existing customers are weighing their options. Meanwhile, Microsoft has delivered a well-received (by critics) mobile OS that is still struggling to find traction in the marketplace.

Of all of the changes that surprised me, Windows Phone 7 tops my list; Microsoft managed to not only deliver an outstanding user experience, but completely reboot its development ecosystem and revitalize it at the same time. What mobile OS surprised you most in 2011, and why?