Right-click a Start Screen tile and a context menu appears. Launch the Start Screen and you'll find new 'Power' and 'Search' buttons, top-right: you can now sleep, shut down or restart your PC in a couple of clicks. You can also toggle this setting manually, as before (right-click the taskbar, select Properties > Navigation, select 'When I sign in.
This starts at boot time, with desktops and laptops now booting to the desktop by default. There are security updates, performance enhancements and other low-level tweaks, but the most obvious changes are to the Windows 8.1 interface.
Windows 8.1 Update 1 is a set of updates and improvements for Windows 8.1.