We've seen most of these features in Microsoft's "first look" video (embedded above), but today they gave us a closer look at all the new features in 8.1. Most of the new features are specific to Windows' tiled "Modern" interface, but there are one or two updates for desktop users as well. Here's what you'll see.
Better Organization and Customization on the Start Screen
The Start screen has a few improvements. You get two new tile sizes: one small square one and one large one, so you can configure the Start screen a bit more like Windows Phone 8. You can also select multiple tiles and put them into a named group, and swipe up gesture for the "All Apps" view. The All Apps view has a few new organization methods, too—you can view them by category, most used, and date installed, not just alphabetical.
The new Start screen has more colors to choose from, and you can even put your desktop wallpaper behind the Start screen as well. You can also turn the lock screen into a photo slideshow, sourced from your PC or SkyDrive. While you're on the lock screen, you can launch the camera or answer Skype calls without logging in.
More Powerful Multitasking
One of Windows 8's coolest features is the side-by-side window snapping, and Microsoft has made a big improvement to this feature: now you can resize those snapped windows however you want. Before, you could only have Windows split 50/50, or into thirds. Now, you can actually drag the slider to make each app take up as much or as little space as you want.
Furthermore, you can have more than two apps or windows on-screen at once—in fact, you can have up to four, as long as your monitor is big enough (Engadget reports that the Surface Pro is still limited to two). You can also move them between monitors, if you have more than one.
Improvements to the Windows Store and Built-In Apps
Windows 8.1 also comes with some handy improvements to the Windows Store and its built-in apps. The whole store has been given a facelift, and it will now automatically update your apps unless you're on a metered connection. Internet Explorer 11 now has unlimited tabs, the camera has a panorama feature, and the new Mail app will have a "sweep" feature that deletes multiple emails of the same type (e.g. newsletters). All apps are supposed to be faster, and push notifications are easier for developers to implement, so hopefully more apps will support them.
Microsoft has also made a big update to search in 8.1 If you open the search charm, you'll see that all your search results are grouped into one place: no more switching between files, settings, apps, and the web. If you press enter, you'll be taken to a full-screen view of your search results. If Bing understands the person, place, or thing you've searched for, it'll load a full-screen app-like view called "Search Heroes," with intelligent results similar to Google's Knowledge Graph, that offers photos, videos, and relevant facts all in one unified interface.
Laslty, SkyDrive is even more integrated with Windows 8 now, automatically updating with new files in the background (like Dropbox) and staying in sync with all your other devices.
Boot to Desktop and the Return of the Start Button
The desktop didn't get as much love as we would have liked, but there are two features desktop enthusiasts have been asking for: boot to desktop and a Start button.
Boot to desktop does exactly what it sounds like: You can tell Windows 8 to boot straight to the desktop instead of going to the Start screen first.
They've also added the ability to shut down from the Win+X menu. Just press Win+X or right-click on the Start button and choose Shut Down from the menu. This is much faster than Windows 8's previous method of shutting down, which required you to open the charms menu. Again, if you use a Start menu replacement tool this won't affect you much, but it's nice that they've added it.
All in all, it's a good update if you're using Windows 8 on a touch-enabled device, but desktop users probably won't find anything too exciting in this update—after all, if you wanted the Start menu back, you probably have it already.