New Start screen live tiles. In addition to the Win8 square and doublewide rectangular tiles, Win8.1 adds quarter- and quadruple-size tiles (see Figure 1). An individual tile's background colors can be changed if the tile isn't "live" (in theory -- I couldn't get it to work in the beta).
Figure 1. Metro now offers four different tile sizes.
New Start screen backgrounds. The very limited set of Win8 Start screen colors and swirls have multiplied enormously, and some of the new ones move. You can set a picture as a Start screen background or specify a set of folders for a slideshow. You can even copy the old-fashioned Desktop's background and use it on the Start screen. That's supposed to make the transition from Desktop to Start screen less jarring.
Better control over grouping and naming tiles. In Win8, it's easy to drag a tile out of place and hard to assign names to groups of tiles. Win 8.1 adds a Customize option (right-click the Start screen or drag up from the bottom) that unlocks tiles so that they can be moved about. You can also type in a Group name without zooming out.
All Apps alternative views. In Win8, the Metro All Apps list shows you a giant blob of tiles, sorted in alphabetical order by the first name of the app, with a concatenated list on the end, grouped using the names that would have appeared in a Windows 7 Start menu (see Figure 2).
Figure 2: The All Apps list in Windows 8 is an amorphous mess.
In Windows 8.1, you access the All Apps list by clicking or tapping on the up arrow on the Metro Start screen or by swiping up. The list can be sorted in several useful ways (see Figure 3). Oddly, though, when you install Office 2013, the Office apps don't appear in the main By Category list.
Figure 3: The All Apps list in Win 8.1 is sortable.
Fewer tiles automatically appear on the Metro Start screen. In Win8, when you install an old-fashioned desktop program, any entry that would have appeared on the Win7 or XP Start menu instead appears as an individual tile on the Metro Start screen. What a mess. In Win8.1, those newly installed programs appear on the Apps list, not on the Start screen (see the tiles marked New in Figure 3). To get the tile to appear on the Metro Start screen, you have to right-click or tap on the All Apps tile and choose "Pin to Start screen." That can be a pain in the neck if you don't know the names of the programs you installed, but it definitely beats having hundreds of useless tiles (InfoPath Filler 2013? Dropbox Uninstaller? Logitech Camera Controller?) squatting on the Metro Start screen.
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