Windows Phone 7 is quite an interesting addition to the mobile OS world. Many mobile operating systems, such as iOS4, Android 2.2, WebOS, Bada, and Symbian have very similar designs. Home screens, icons, widgets, siloed applications, they're all fairly prevalant across the current existing mobile operating system landscape. Windows Phone 7 has taken a different approach with tiles, Microsoft's way of mixing widgets with icons. The tiles are large and easy to read, as well as easy to select.
When you touch a tile, it does a slight tilt, when you reorient the phone, there's a slight animation, these small touches really make the HTC 7 Mozart stand out. Windows Phone 7, after a bit of playing around, is very easy to understand and easy to use. The tile approach is actually very effective in giving you loads of information but not overloading you to the point of where your screen is jam packed.
Another introduction with Windows Phone 7 are panes. Panes stretch across the phone and group together similar types of information. You'll have to drag the screen around to see all the information and at first, it's a bit hard to swallow. It does take a bit of time to get used to, but it is an interesting approach.