A Failed Idea in Windows
Here's a failed idea in Windows: Active Desktop. This feature has existed in Windows 98 and remained, though gradually pared down, all the way to Windows XP, and possibly Windows Server 2003, but it won't be in Longhorn.
Created in the days when Microsoft felt threatened by Internet, Active Desktop essentially turned the desktop into more than a mere icon and folder holder. It turned the desktop into a web page. In addition, the background of any folder could also be turned into a webpage.
Windows 95 didn't offer great support of the Internet, but Microsoft was determined to redesign Windows 98 to provide superior integration with Internet. IE support was directly integrated into the shell browser. Windows 98 included a Personal Web Server. MSPaint was upgraded to support JPG and GIF, in addition to BMP. Windows questionably offered a mode, when selected icons can display underlines, and single-clicking, not double-clicking, can launch icons. I distinctly remember that either FrontPad or FrontPage Express was available in an early version of Windows 98. Then, there was Active Desktop.
A good amount of resource and attention was expended on this feature, but, in the end, few customers used this feature or even knew about this. Active Desktop was motivated by the current interest in push technology, symbolized by PointCast and Netscape channels.
While Longhorn no longer supports Active Desktop, I think it provides a more well thought-out and baked-in approach to supporting Internet concepts. Longhorn includes its own declarative UI markup language, XAML. A set of XAML pages with code can be stored within a single .container file using the OLE compound document format. A XAML page can then be displayed through IE (which hosts active document control for XAML) or compiled into an existing application. One could imagine that Internet websites may even deliver direct XAML to Longhorn clients, instead of HTML, for a richer experience.
Longhorn natively supports a new application type called navigation application. Longhorn applications and pages can launch from the web into a secure sandbox using .NET code access security mechanism. Longhorn introduces many of the advantages of the Web world into Windows application development such as simple deployment and automatic updates.