Fast and fluid animations bring apps to life. In Windows 8 Consumer Preview, you will notice that animations are an integral part of the user experience. When you log in to your PC, the items in the Start menu animate in. When you launch an immersive app or zoom into the Start menu, the experience is enriched with a smooth animation. Animations can tell the user what happened as a result of a specific action. This visual feedback instills user confidence in the app’s responsiveness. Even a simple addition or deletion of an item in a list can be made fluid, modern, and informative using a subtle animation (as seen in Jensen Harris’s //build/ session, around minute 25:00).
Fast and fluid animations in your Metro style app - Windows 8 app developer blog - Site Home - MSDN Blogs