Part of the appeal of Google Android and Apple IOS is that they are task-centric, with lightweight tasks (apps) designed to be suspended and restarted quickly and seamlessly. When you're in (say) Gmail in Android, you can touch Home and then (say) Maps to quickly switch to that task, with Gmail suspended in the background consuming relatively few system resources, and if Android needs those resources, it can kill Gmail while saving state information for a fast restart. If you then touch Home and Gmail again, you are right back where you left off in Gmail whether Gmail is being awakened or restarted. You never have to close an app. There's really no way to do this in Windows without a huge performance hit and/or a huge amount of re-engineering that isn't going to happen. Plus cloud-centric computing is already built into Android, and now into IOS as well (the biggest part of the iPhone 4S announcement).
Another part of the appeal of Android and IOS is the ability to easily merge disparate sources into one UI component; e.g., a widget that merges phone (Google) contacts, Facebook contacts, Twitter contacts, etc. Windows 8 is an attempt to do this on the PC, but since it fails to solve the app-centric paradigm issue, I don't think it's going to be all that successful -- Windows users have too much invested in current apps and how they work. What I think Microsoft really needs is a new cloud-centric desktop OS (like Chrome), but the Windows 8 Preview isn't even half that loaf. Microsoft may now get it, but apparently lacks the courage to do it.