Looking at the key features published on the Windows Team blog by Brandon LeBlanc, there's one thing that one cannot help to notice. Windows RT could be for Microsoft what Snow Leopard was for Apple, the chance to break away from legacy applications altogether and one some of us - including myself - will be ready to embrace wholeheartedly.
Windows RT (note the lack of numerals) cannot be upgraded from previous Windows OSes (Windows 7 and 8). Like Windows Phone, it will come with Microsoft Office but won't allow installation of x86/64 and other desktop software.
Oddly enough, there's no Storage Spaces or Windows Media Player - which could mean that Microsoft preclude the use Windows RT as a media-centric OS - but you do get Device encryption by default and the rest of the key features found on Windows 8.
Perhaps the biggest difference of them all is that Windows RT will not be available as a stand alone, shrink wrapped package and will only be available on PCs and tablets (possibly other devices as well) pre-installed.
Installing new apps on Windows RT will only be possible - you guess it - as apps via Microsoft's own app marketplace called Store.
Source: Windows Team Blog