As you know by now, Android followed the iOS approach by making the Android platform app-based. That is, the Android homescreen can be populated with as many Android apps as you can possibly fit in addition to widgets and shortcuts. Depending on your Android phone, you can have as many as seven homescreens filled up with Android apps, widgets and shortcuts. This approach proved successful for the Android platform and this is the bread and butter of Apple's iOS.
Interestingly, Microsoft decided to take a different approach to the Windows Phone 7 homescreen. Instead of having multiple screens filled with apps, Windows Phone 7 only has one main screen containing functional hubs or tiles which are interactive as well. By this we mean, depending on what these tiles contain, they are refreshed wirelessly via the web. You pin appointments, news and weather, apps and other information on these tiles to make them functional and useful. This is somewhat similar to the Android widgets which you can add to your Android phone's homescreen.
Obviously, Microsoft doesn't want to ride on the popularity of the app-based ecosystem. While this is a "fresher" approach, users who are planning to transition from the Android platform to Windows Phone 7 might find this a little bit off-tangent and it could discourage them to continue with the shift.
On the other hand, this could also encourage some smartphone users, especially those who want a more professional looking homescreen than what Android has to offer.