I recently stumbled on YouOS and was struck by how bad an idea I thought it was. I don't even have to write down why, because Jon Udell has already beaten me to the punch with his article Application UI goes back to basics where he writes

Consider the effects of the graphical user interface. At hospital admitting desks, in accountants’ offices, and at video retail stores, I watch people perform tasks for which the desktop metaphor — with its cluttered surface and overlapping resizable windows — is at best a distraction and at worst an impediment.

Although YouOS is an interesting bit of technical wizardry, it seems like a step back when it comes to providing value to end users. The fact that there are multiple, tailored interfaces to my data on the Web (e.g. del.icio.us for my links, My Yahoo! for my digital dashboard, MSN Spaces for my photos and social network, etc) all accessible from a different tab in my browser is a lot more powerful than the classic WIMP interface that drives desktop computing. Trying to port the desktop metaphor to the Web is like working on how to fuel your car with hay because that is what horses eat.

Last year at the Web 2.0 conference, both Ray Ozzie and Sergey Brin said similar things when asked about Web-based office suites. Of course, since then Google purchased Writely and shipped Google Spreadsheets which is somewhat contradictory. :)