On the opening day of its Build 2013 conference in San Francisco today, Microsoft unveiled its Windows 8.1 Preview, the first and only public milestone release of its first major update to both Windows 8 and RT. Billed as a more complete look at Microsoft’s vision for the future of computing, Windows 8.1 adds many new features to Windows 8/RT while fixing numerous customer complaints as well.

“The biggest feature of Windows 8.1 is that it only took us 8 months to get here,” Microsoft corporate vice president of Windows and Windows Services Julie Larson-Green said during a pre-Build briefing. “We’ve changed the way we do engineering and are quicker and more responsive to meet the needs of more rapid technology change, our customers, and our partners.”

To that end, Windows 8.1 represents both what the Windows team can accomplish in one year—previous releases were generally three years apart—and what this team originally envisioned for Windows 8/RT. It is more complete, more cohesive, and more broadly usable on a range of different device types than was the original release of Windows 8.

I’m writing a lengthy series of articles about my experiences using the Windows 8.1 Preview on a number of different PCs and devices, so be sure to check out Hands-On with Windows 8.1, which will grow over time to encompass more of the new features. But here’s a quick peek at what’s changing.

Windows 8.1 has been adapted to make the OS more usable on a coming generation of mini-tablets with 7- and 8-inch screens, says Jensen Harris, a key member of the Windows User Experience team. Its updated Start screen supports multiple tile sizes and the system better supports portrait mode, common in mini-tablets, and not just for reading: All of the built-in apps are being redesigned to accommodate this new mode. And the touch keyboard has been redesigned to support child keys, allowing those using mini-tablets to type more effectively in portrait mode.

There are two marquee new features in this update: Smart Search and SkyDrive integration. Smart Search melds local searches for apps, files and settings with web results, providing Windows 8.1 users with a single place to go to find virtually anything, and while it sounds overloaded it is in fact in use quite elegant. (Microsoft has also changed app behavior so that most built-in experiences offer their own contextual search without cluttering it up with other search types.) And in this release SkyDrive is thoroughly integrated into the system, with immediate access to all of your files and far more syncing between PCs and devices. “Windows is powered by SkyDrive, and is powered by the cloud,” as Harris said.

Many of the built-in apps are getting updated. The Photos app is almost unrecognizable and now sports surprisingly elegant editing features. Xbox Music (but not the other Xbox apps, yet) has gotten a pretty new interface that reduces the number of clicks it takes to get to your own music. And the SkyDrive app can now browse your PC’s file system too. It’s not in the preview, but a coming update to the Mail app is a stunner and appears to offer all of the functionality users have been requesting, including drag and drop and linked inbox.

There are new apps, too, and many, many more settings in PC Settings, allows more people to stay in the new “Metro” environment while using modern tablets and hybrid PCs. A Reading List app saves online articles for later reading, and syncs across your PCs. New Bing apps like Fitness & Health and Food & Drink, the latter of which supports a “hands-free” mode for use in the kitchen—just wave your hand in front of the PC and the recipe will advance to the next page—are particularly attractive.

The most surprising revelation so far this week, however, was a confirmation that Microsoft would bring its hugely successful Office franchise to the “Metro” environment in the near future. There were no promises about a delivery date or any practical information about licensing. And the quickie PowerPoint demo we received gave no hint at all about the editing capabilities, being described as “alpha code.” But it was stunner and generated a surprising amount of excitement. “This is Office for the Modern user experience,” Harris said, swiping through a PowerPoint presentation to cheers.

This will be a busy week, so please stay tuned to the SuperSite for Windows for the latest developments. If you’re interested in installing the Windows 8.1 Preview—for Windows 8 or RT—please visit the Windows 8.1 Preview web site.