Converged apps and UIs
This approach of divorcing the endpoint from the user's data and services goes beyond file state, as Mountain Lion so clearly shows. There are three fundamental advances in Mountain Lion, which when all is said and done is just an incremental update to 2011's OS X Lion. One is the deepening of the cloud storage architecture. Another is the deeper integration of social network (more on that later). The third is the convergence of users' "personal" apps between iOS and OS X.
Apple has ported its iOS-only Reminders (task list), Notes (note taking), and Messages (instant messaging) apps, as well as its iOS-only notification engine to OS X Mountain Lion. It's also renamed the Mac's Address Book to use iOS's Contacts and renamed the Mac's iCal to use iOS's Calendar. Changes to these apps in Mountain Lion suggest richer capabilities to come to iOS later this year -- Apple cross-pollinates OS X and iOS regularly.
At a technology level, these are minor changes, to be sure, but they make the user experience across smartphones, tablets, and Macs even more similar. In Lion, Apple's done an amazing job of bringing iOS's touch interface to the Mac, and Mountain Lion extends that. As someone who works mainly on a Mac and iOS devices, I can tell you that the "switching" cost as I go from device to device diminishes with each OS X and iOS rev. It's becoming a single operating environment that adapts to its specific devices -- that's exactly the point.
Microsoft has the same approach as Apple, even if it's earlier in that journey. Metro runs on all Windows 8 devices, so it'll be on desktop PCs, laptops, and tablets. It's already on Windows Phone 7 devices. It's the "BigWin" Windows 7 operating environment that is not universal in Microsoft's path: x86-based PCs, laptops, and perhaps tablets can run it, but that's all.
This split goes beyond UI; because Metro can't run the kinds of apps Windows users expect, applications on x86 environments differ significantly than the widgetlike Metro apps seen so far. Microsoft promises a redesigned, "full feature" version of Office for at least ARM-based Metro devices, and its WinRT development tools may help developers deliver "real" apps to Metro.
Contrast that to the increasingly desktoplike apps available on iOS, in addition to the thousands of widgets in the App Store. Let's hope Microsoft gets "real" apps on Metro; if Metro stays stuck in widget land, Microsoft will have defined its mobile devices as weak, and that will keeps its mobile market share extremely low as users continue to choose iOS or Android "strong" mobile devices as companions to their Macs or Windows PCs.
It's clear that Windows 8 is nowhere near as integrated across devices in terms of UI and apps as iOS X is. Instead, what you have right now are two parallel operating environments that have very little in common. I can vouch that switching between them is a huge psychological adjustment. That'll make the initial Windows 8 experience difficult for most users, but I suspect Microsoft sees Windows 8 as the pivot point in its OS development, and the Metro UI will start to absorb the traditional Windows interface in future versions. Microsoft has to move fast: Apple has been plotting its convergence and executing it step by step for five years, whereas Microsoft started in a serious way only last year.
Social federation, not just device federation
iOS 5 integrated Twitter, and Mountain Lion adds that, along with several other social services to its standard supported accounts. Microsoft has been far ahead of Apple in social networking integration, of course, on the smartphone side and is bringing that integration to Windows 8.