Microsoft bloggers Steven Sinofsky and Chaitanya Sareen explain the advantages of Windows 8's new Metro Start screen and why it offers a better user experience than the old Start menu, you can read it over here.
Based on these challenges and your feedback, we’ve continued to refine the Start experience over the years. However, we find that even the Windows 7 Start menu still faces core usability challenges:
The menu feels cramped relative to available screen real estate when you try to see and navigate the full catalog of your programs.
Search doesn’t have the space it deserves to quickly show you rich results across all sources of information, especially on larger screens.
It’s hard to customize the menu to make it feel like it’s really yours.
Icons and shortcuts are static and don’t leverage more of the pixels we see in modern graphical interfaces to surface connected scenarios.
Of course, the above list isn’t a complete set of everything we are improving. We also aim to unify Start with the rest of the system and enable new scenarios. An important part of design is sometimes taking a step back to fundamentally reimagine something from the ground up in order to bring more than incremental improvements to a product. This is especially true for something like Start that was born in a very different time when we didn’t use our PCs the way we do today.