Windows 8 is a brand new interface for tablets. It looks a lot like Windows Phone Live Tiles.
All apps can be viewed in this tile-based UI, which is, very impressive. You can totally see the Windows Phone influence on the interface, from the Live Tiles (which show info without having to open the app) to the touch keyboard, which even has a "thumbs" mode.
The animations and gestures and multitasking are all pretty damn smooth looking, as you can see in this demo video (or you know, up top). Swiping from the right reveals the taskbar, with the Start button, search, share and settings. Swiping from the left switches between apps, the background application zipping in and unfurling like a magic carpet. Snapping lets you have two apps next to each other, simultaneously. The main app takes up most of the screen, while the secondary app is like a docked sidebar; you can swap which you've got snapped very rapidly by swiping from the left to cycle through background applications.
The idea of running real, full Windows apps on a tablet (or anywhere) isn't a bad one—in fact, a ubiquitous OS that'll run on any device, perfectly, would be totally amazing—but mixing the two looks kinda gross and weird in practice, at least in its current state. Maybe the way Microsoft's gonna translate touch apps to keyboard/mouse and keyboard/mouse apps to touch to achieve this infinite OS will be downright magical in the end. But I'm not seeing this hybrid thing right now, even as impressed as I am by all of the incredibly cool modern interface stuff that's totally designed for tablets. (I really, really want that stuff on a tablet, to be clear.) And beyond that, at least when we're talking about tablets, it looks like Windows 8 still has a lot of the rest of the problems that made the current Windows less-than-good as a tablet OS—or it doesn't have the things that makes the other tablets as good as they are. Namely, utter simplicity. This. Is. Windows.
That said, we'll apparently see a lot more in September at Microsoft's Build conference. So I'm still hoping to be blown away as we get deeper inside Windows 8.