The magic is in the Multi-Touch.

The new Magic Mouse redefines what a mouse should do. In addition to its smooth, seamless top-shell design that acts as one button or two, Magic Mouse features a Multi-Touch surface with gesture support. Scroll vertically, horizontally, and diagonally — a full 360 degrees — simply by touching anywhere on the top surface. With a greater surface area for scrolling, you can get around a long timeline in iMovie, through a lengthy web page in Safari, or around a set of images in iPhoto more efficiently. You can also swipe through pages in Safari or photos in iPhoto with two fingers. It’s the most advanced Apple mouse ever made.