During the Build keynote, Microsoft also officially unveiled Windows Phone 8.1. Most of its features have long been known, so I'm not going to go into all of them in detail, but suffice it to say this is a huge update. Microsoft focuses a lot on its Google Now and Siri alternative Cortana (The Verge has a great article on it), which works more or less in the same way, but with one interesting strength: integration with third party applications.
Windows Phone 8.1 will become available for developers this month, and will be pushed to current devices in the coming months. It will also be available on new devices during that same timeframe - and it'll arrive on all Windows Phone 8 devices (every time a Microsoft employee points this out, an Android 2.3 device explodes).
I am very psyched for this massive update. It might not make much of a difference in the marketplace, but that doesn't really matter for me personally. This simply looks like a fantastic update, and I can't wait until my developer-ready HTC 8X gets the developer update.