The shift to Windows NT as a base for Windows Phone brings lots of improvements with it. First and foremost, multicore support is a core aspect of Windows NT, and phones will benefit from it - finally, multicore Windows Phones. The driver model and several frameworks are also shared between Windows 8 and Windows Phone 8, which should make driver development a little bit easier for hardware makers.
Windows Phone 8 will also have improved resolution support; the maximum for WP8 will be 1280x768, with three supported resolutions total. Applications do not have to be adapted to support these new resolutions, as they will just work. You can, of course, alter your applications to make optimal use of the increased number of pixels.
Another big new thing for the Windows Phone world: native C and C++ code development. This was a huge gap in Microsoft's offering which will now finally be plugged. Thanks to sharing the graphics framework with Windows 8, it'll also have DirectX - in other words, game developers are going to love this stuff for easy code sharing.
There's lots more in Windows Phone 8: cool NFC stuff for digital wallets and cross-device information sharing. Nokia Maps will be integrated into WP8, including turn-by-turn navigation and offline support. A big finally: hot-swappable microSD cards. Yes, we're hitting state-of-the-art here.
The most user-visible feature they unveiled was what Microsoft calls the 'new' start screen. It's not really new though; all it adds is a third tile size (so you can stuff four small tiles in the space of a single larger square tile). A very welcome change, but I don't consider such a small change to justify calling the start screen 'new'.
A big change is coming to how updates are handled. All updates to the operating system will arrive over-the-air, so no more silly USB tethering with the Zune application. In addition, starting with Windows Phone 8 (kind of crucial), every device will be supported with updates for at least 18 months after launch of the device.
Now comes the bad news. Existing Windows Phone 7 users will not get this update. Windows Phone 8 requires brand new hardware, and none of the current phones - not even the fancy Lumia devices - will get Windows Phone 8. Instead, us WP7.5 peasants will have to settle for something called Windows Phone 7.8, which basically just adds the smaller tiles. According to Microsoft, it will sport more - but they didn't reveal anything, so it can't be much.
I'm incredibly lucky - my HD7 was a gift 18 months ago, and I didn't spend a penny on it. However, those other three guys and the cow, who bought expensive new Lumias, just got screwed. I was expecting this - but still, I'm very, very disappointed. This is way, way worse than Apple and iOS 6 Starter Edition, iOS6 Home Edition, iOS6 Professional Edition, and iOS6 Ultimate. Not sure if it's better or worse than Android's mess on this front.
Windows Phone 8 looks awesome, but the update policy is a disgrace. Who in their right mind is going to buy a Windows Phone device now? For what it's worth, Windows Phone 8 will arrive somewhere in the Fall - probably alongside Windows 8.