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Gmail support is still limited
Last year, shortly after Windows 8 was released, Google abruptly discontinued support for Exchange ActiveSync (except for paying Google Apps customers). As a result, the Windows 8.1 Mail app uses IMAP to sync messages but doesn't offer the same sync support for contacts and calendar items as it does for Outlook.com (Hotmail), Office 365, and other Exchange-compatible accounts.
One of the signature features in Windows 8.1 is a major expansion of the built-in search capabilities. Click the Search charm to open a search pane on the right. Enter a search term and some results appear immediately beneath your input. If you don't find what you're looking for, click the search button (or press Enter) to see a complete list of results, including installed apps, settings, and results from the web.
If you're firmly convinced that Google is better than Bing, you can disable the Bing results and just use the search tool for local files, apps, and settings. But I've found the results generally excellent, and pressing the new system shortcut for Search, Windows key+S, is now second nature for me.
Enhanced multi-monitor support
Windows 8's support for multi-monitor configurations was severly limited, with no ability to mix immersive apps and desktop apps on a display. Windows 8.1 vastly improves this capability. You can now snap immersive apps (a Twitter feed, tracks in a music playlist, your email message list) alongside desktop apps on any monitor, with the ability to show the Start screen on either display.