Aside from the new emulator looking beautiful in design, there's many changes to the base OS (hinting changes at the final WP7 build, too). Originally, you had a choice of only 4 colors: Red, Green, Blue, and Orange. Now, you have the ability to choose 10 different colors, as well as a light or dark background.

Color choices:

• Magenta
• Purple
• Teal
• Lime
• Brown
• Pink
• Orange
• Blue
• Red
• Green

While this is a nice touch, it'd be even better if we had the ability to choose, say, a custom wallpaper for the homescreen.