Windows 8.1 is now available free for existing users of Windows 8, and in a break with tradition, will not be sold as an upgrade to existing installed Windows versions – it’ll just be the full version, and that’s it.
Upgrading from Windows 8.1 Preview isn’t officially supported – though it can be done (you do need to match the language version exactly – if you installed the Preview as English US, you’ll need the English US ISO from the MSDN etc site, to upgrade).
There are plenty new features in Windows 8.1, as well as the much-documented return of the Start button (after all the fuss about the removal of the Start button from Windows 8, and the subsequent rush from application vendors to restore it, there’s already a Start Killer app for Win8.1 which removes the reinstated Start button… you can’t win, sometimes…)
There are a bunch of new apps with Windows 8.1, and major improvements to existing ones. One new app of some interest is the Reading List: the idea being that if you’re looking at web sites in the IE11 browser (the Modern UI version) or possibly reading content in other apps (like the Store), if you flick the Charms out and choose Share, then you can add the site/content link to you Reading List.
Start the Reading List app up, and it will show you the list of sites you’ve bookmarked, and also make use of Windows 8.1’s improved side/side view, to show the content shown alongside.