The key here is "compatible." Microsoft's many previous attempts to gain a presence in the tablet market -- it's been trying for a decade longer than the iPad has existed -- have failed because its Windows OSes weren't designed for the context. Slapping a pen- or touch-based interface onto an OS designed fundamentally for mouse and keyboard input didn't work. Apps, common libraries, and OS services all assumed those input devices, and what they presented users onscreen was simply unsuited for pen or gesture control. A few industries created special tablet apps that were pen-oriented, but in essence that reduced Windows tablets to dedicated app devices, keeping it out of broad-based usage. By contrast, Apple designed iOS for gesture-based input, and people took to it easily -- and developers had to think in a gesture mind-set to even be able to deliver apps.
From the hints that Microsoft has dropped, it appears that Windows 8 is designed for gesture-based input, that its UI is not the standard Windows interface with some touch technology slapped on. In fact, Microsoft's limited showing of the Win8 UI thus far indicate it's based on the compelling Windows Phone 7 UI. If this is all true, and the touch UI is not just skin-deep, Microsoft may have finally removed one of the key barriers to successful Windows tablets.
But it risks breaking the experience on the desktop, where the vast majority of people still compute. (Although I believe we are moving into a post-PC world in which mobile devices will become dominant, that won't happen overnight, and there will still be a place for old-style PCs as specialty workstations.) Microsoft's pitiful touch-based version of Windows 7 showed how touchscreen PCs make no sense -- the screen is in the wrong place, for starters.
Microsoft has hinted that users will be able to use the legacy (Windows 7) UI on the desktop if they choose, which could either allay these context issues or create a confusion as some apps use the old UI and new apps use the Win8 UI. Microsoft has to figure out how to have a common UI on each device at least. It's fine if the UI differs between tablets and PCs, as they are used differently and thus should have variations. Apple has shown that beautifully in iOS and Mac OS X, which share UI approaches but remain distinct where it makes sense. Microsoft has to reach similarly smart decisions.
The other key piece is knowing when not to be compatible. An iPad can't run Mac OS X apps, and Apple seems to have no plans to make that happen. That has let iPad apps be focused on the iPad's capabilities, not be poor ports -- the kind often seen in Web apps designed for universal deployment across PCs and mobile devices. It appears that Microsoft has reached the same conclusion as Apple: Tablets won't run legacy Windows applications, so new applications are more likely to be designed afresh to be good tablet apps.