Most iPhone developers I know find that learning Objective-C from the NeXT operating system to be a daunting task compared to modern, high-level languages like C# and VB. While the world is full of Java developers, the complexity of cobbling the necessary tools together needed to build for Android apps is a real productivity killer. Just running Eclipse on JDK 1.6 sucks the life and performance out of my fast Windows 7 laptop. Microsoft Visual Studio 2010 Express for Windows Phone is free and the emulator + SDKs all download and install together making the whole process fast and simple. Apps get access to all phone sensors, a local database (SQL Server Compact), and Metro design.
Better productivity means faster time-to-market which means more apps for Windows Phone.
As we've noted here on AAWP many times, more applications are being submitted to the Marketplace every single day - and while it might not have numerical parity to Android or iOS, Microsoft are doing exactly what they did with the Xbox and Xbox 360 developer tools and keeping them as simple and familiar as possible to those working on their platform. That strategy looks to be working on Windows Phone, and as more developers find out how easy the jump is to make, the submission rate can only increase, which is great news for everyone.
Hat tip to Windows Phone Thoughts.