I Have spent the last 6 months or so working on WP7 and just wanted to share some thoughts about it.
When Windows Phone 7 was first released the platform was quite immature. There was no multi-tasking or background services and the browser was not very good along with some smaller issues. With the Mango update both of these larger issues were rectified and the platform has caught up with Android and iOS quite quickly in other aspects such as voice input and platform search.
The development tools are one area where Windows Phone is strong. Visual Studio is a mature and stable development environment. The programming language used is C# is powerful and easy to pick up if you know an OO language. Expression Blend can be used to create a polished, sophisticated User Interface (UI) using XAML, an XML markup specifically for UI. Games are created using XNA, the same tool set that is used for X-Box games. This means that same code base can be used between X-Box, Windows phone and Windows, creating a seamless, cross platform experience where the same game can actually be played across all platforms. One drawback is that Windows 7 is required for development as Visual Studio 2010 will not work on Windows XP.
The ecosystem appears to be more like Apple than Android. For example, there is an annual subscription for publishing apps on the market place rather than a small one off payment. Without this subscription it is not possible to test on real devices, only on the emulator in Visual Studio. The apps also have to go through a quite stringent submission process, however in the latest version of the development tools, there is a testing suite to help you to get through this without any problems.
The tools for Windows phone development are strong and the platform is slowly(quickly?) catching up with the Android/iOS. My main concern right now would be consumer acceptance and who this device will appeal to. The consumer market is dominated by Android/iOS and other platforms are going to struggle to take market share from them and this is the market that WP7 is aimed at. Microsoft have moved away from the enterprise market who seem to prefer Blackberry at this moment in time due to the security of the platform.
One area that might be worth keeping an eye is the games market and the cross platform code. Maybe this could be applied to the rest of the platform in the same way to make it more compelling. Apps would look and behave in almost the same way on each device with the data stored in the cloud.