Well, I have to disagree with the premise..it's not just about the interface. The interface is a very important part of the equation. But it's about the total experience. If it was just about the interface, everyone would have switched to a Mac along about the Windows ME days. So far Microsoft has done well with the interface, but the overall experience still lacks both the iPhone and Android as far as I'm concerned. Part of this is due to their partners treating it as a 2nd tier device and not matching up hardware wise. But the other part is the the lack of depth or choice in applications, often coupled with poor third party app performance. The OS is still somewhat limited for my tastes.
Microsoft still needs to move FASTER if they are going to be considered a major contender in mobile phone space.
As far as Android is concerned, my wife, who is anything but technical (seriously more than one remote for the entertainment system and she just gives up), moved from a feature phone to Android with little issue, except trying to use the onscreen keyboard. A quick install of a third party keyboard resolved that issue. I'm sure WP7 would have worked equally as well for most her needs, but she would not have had the option to load a third party keyboard if the WP one was difficult. The point is that Microsoft seems to be pointing towards this "new user" market and touting the interface experience..but these new users that don't want to do a lot with their smart phones really don't have a lot of problems using any of Android, WP, IOS or even RIM for that matter; for the things they want to do. It's just not that hard on any of them. They all have easy access to email, the web, and social networking.