B2G, unlike Palm's WebOS and Microsoft's JS+HTML5 Metro platform, is a genuine Web stack with DOM APIs to all the hardware capabilities. You don't use proprietary libraries and frameworks to get at the telephony stack or the vibrator or the camera or the accelerometer or the bluetooth stack or USB or any other hardware device. You use JS and the real DOM. All standards-based and of the real Web. These capabilities, these new DOM APIs will make it into the standards and be available from browsers and other real web stacks across all platforms and all devices (where the capabilities exist.)
It should be noted that the UI was completely revamped before MWC. A ton has changed since these screenshots.<p>(Disclosure: I'm a Boot2Gecko hacker for Mozilla)