Windows Phone 8.1 adds compelling user-facing features, but it's a bit of a mystery just how much developers will be able to take advantage of them. The revised smartphone operating system from Microsoft will hit the street as soon as May in new hardware, and will be distributed to existing handsets by the summer months.

Perhaps the biggest hit of Build's day one keynote was Cortana, Microsoft's new virtual personal assistant. Cortana of course takes on Apple's Siri and Google's Google Now in that it listens to and responds to natural language requests and commands. Microsoft VP Joe Belfiore was delighted to walk the 6,000 attendees at Build through all the different things Cortana can do. Cortana can set reminders, read emails, dictate text messages, set alarms and calendar appointments, search for nearby points of interest, look up sports scores, and open other apps.

Like Siri and Google Now, Cortana relies on the cloud. It listens to spoken language, sends the voice recording to Microsoft's servers where it is translated into text, and then performs the requested action. During the on-stage demos, Cortana worked about three-quarters of the time. It failed to understand Belfiore's pronunciation a few times, which is probably reasonable behavior for a beta service.

Cortana is one of the few new things in Windows Phone 8.1 that developers will be able to fool around with. Microsoft said APIs will be available to developers so they can hook their apps into Cortana, though Microsoft cautioned that functionality will likely be limited until Cortana graduates to a full release.

The lock screen is the only other aspect of Windows Phone 8.1 that Microsoft specifically said will come with a new API. In Windows Phone 8.0, the lock screen is rather static. It presents some notifications and a rather boring time/date stamp. The new lock screen is fully customizable. Microsoft plans to release an API that will let developers create and sell their own lock screens. Some of the sample lock screens shown by Microsoft during the keynote were quite attractive. Giving developers the ability to craft unique lock screens means, in the long run, Windows Phone owners will be able to further personalize their devices.

Beyond these new features, Windows Phone 8.1 also adds new views to the calendar application; an actionable notification bar; burst shooting in the camera; simplified Photo, People, and XBox Hubs; a swipeable keyboard; three columns for the Start Screen; transparent Live Tiles; and much, much more.

The SDK for Windows Phone 8.1 can be downloaded here.