It's not finished yet, and Microsoft still has plenty of work ahead of it, but one thing is clear: Windows 8 is a genuine, uncompromised tablet operating system.

It was a long time coming. For many years, Microsoft worked in vain to crack the tablet market. Its previous tablet efforts treated the finger or, more commonly, the stylus as a mouse replacement, never recognizing that touch is simply different. The mouse is precise, accurate, but indirect; touch is imprecise and sloppy, but the direct manipulation it affords makes it fast and fluid.

Windows 8 makes touch a first-class citizen. Where Windows 7 penalized mouseless, keyboardless users with a fiddly, mouse-oriented user interface, Windows 8 lets you leave the mouse and keyboard behind.

The interface borrows and builds on concepts that Microsoft first used with Zune and Windows Phone. It uses Microsoft's "Metro" design language: crisp text, simple geometric shapes, parallax scrolling, and no skeuomorphic pretense.

Nowhere is this heritage more evident than on the lock-screen—typically the first thing any Windows 8 user will see. With a big background image, some notification icons, and a clock, it looks much like the Windows Phone lock screen. Slide it out the way and, depending on your security settings, you'll be prompted for a PIN or password—again, just like the phone.

This reveals the centerpiece of the entire user interface: the start screen. The start screen replaces the Start menu. And it does so for everyone; this is not just a touch interface, it's the interface. If you want to launch an application in Windows 8, this is how you do it.

The start screen is built from tiles, the same square and rectangular blocks that populate the Windows Phone interface. Tiles are no mere icons; they're active, living things that update automatically. Unlike the phone, which organizes all tiles into a single block, tiles in Windows 8 can be grouped together. Between each group sit "speed bumps" so that scrolling the start screen sticks momentarily on each group, making it harder to overshoot.

Personalization is a core concept to the start screen. The set of tiles on display, and their organization on-screen, is entirely up to the user. Manipulating tiles to customize their appearance is easy.

Microsoft avoids the modal interfaces found on other platforms. Instead of long-pressing on a tile to switch into a "move tiles around" mode—as used on both Windows Phone and iOS—a tile can be picked up directly just by dragging it up or down. Nudging the tile up or down enables customization of that tile; its size can be changed (either a square or a double-width rectangle), it can be removed from the start screen, or it can be uninstalled entirely.

Dragging tiles around, to move them between groups, leverages multitouch. Pick the tile up with one finger, and you can then scroll the entire start screen with your other hand. The start screen supports pinch zooming, too. Zoom out far enough and the tiles remain readable by switching to icons, a feature Microsoft calls "semantic zoom."

Central to the Windows 8 touch experience are three key gestures: Swipe in from the left, swipe in from the right, and swipe in from the top (or bottom).

Swiping from the left is used for task switching, a feature called the "switcher." As you swipe, a thumbnail of the next running application appears and can be dropped into place. Windows 8 also includes a side-by-side multitasking mode, activated by dropping the application thumbnail onto the left or right edge of the screen. In this mode, the screen is split into two sections. The split isn't flexible; there's one large part and one narrow part, which Microsoft says makes it much easier for developers to ensure their interfaces work well.

Swiping from the right brings up what Microsoft calls the "Edge UI" filled with "charms" (and some overlays the system clock). There are five charms—search, share, start, devices, and settings. Start, unsurprisingly, takes you back to the start screen.

Settings serves dual roles. It shows a set of common notification icons—network state, volume, and so on—and it also contains application settings. This is where applications put their own preferences and configuration.

Search and share are the most interesting. The basic search feature is broadly equivalent to Windows 7's Start menu search feature—searching applications and documents—with a Metro-style appearance. But there's a key difference: applications can hook into the search feature, so that a search can perform some application-specific action. For example, a Twitter client could plug into the search system and allow direct searching of Twitter.

Microsoft calls this plug-in mechanism "contracts." There are several contracts defined: share, search, "print to," "play [media] to," and more (about 12-15 in total). The purpose is to let applications work seamlessly together; instead of isolated silos of data—one app for your photos, one app for your social networking, and so on—contracts allow structured, controlled data sharing.

Share, accessed from the share charm, has two contracts: one for applications that can share data, and one for applications that can use shared data. Internet Explorer, for example, can share data—it can send a link to the current page to other applications. Social networking apps can receive shared data. Pair them together, and you can take a URL from Internet Explorer and post it to your Facebook wall.

Another important contract is "pick." Social networking applications are often chock-full of cloud-stored pictures. But what if you want to attach one of those pictures to an e-mail or a tweet? Windows 8 is a PC operating system, so it does have a local file system with a file browser, and you can pick pictures there, but traditionally that doesn't give you access to those cloud pictures. Instead, you might copy the URL, or even download the file locally and attach it.

Windows 8 has a new option: that social networking application can offer the picker contract, and it will then plug in to the standard file picking user interface. Files picked using the picker contract are given to the application, just as if they were local.

Devices is, obviously, used for configuring devices. Just as with search and share, hardware vendors can create Metro-style front-end software that slots into the Edge UI panel.

Of course, an operating system is nothing without applications. The developer preview currently available has a bunch of demo, proof-of-concept applications, and a handful of real applications, including Internet Explorer 10 and a Remote Desktop Client. Internet Explorer 10 runs as a Metro-style application. Full-screen, edge-to-edge webpages. Swipe the top or bottom edge of the screen and you get the App Bar, with your address bar, stop/reload buttons, and tab switcher. Click back into the browser and the chrome disappears.

The core UI works, and it works well. It's fast and fluid, and it's very well thought-out. Multitasking, personalization, and interconnections between applications are all at your fingertips, and the Metro look-and-feel ties everything together.

The developer build being given out today is, however, very rough. Some demonstrated features, including speed bumps between groups and semantic zoom, aren't implemented in the build. In a few hours of usage, we've experienced regular application hangs and one total system hang. Microsoft isn't calling this a beta, and isn't claiming it to be anything other than a preview, and that's appropriate—this isn't ready to be used as a regular, day-to-day operating system yet.

But even in this early state, Windows 8 is unambiguously a first-class tablet operating system.