Tim Bray: Not an OS:

…I: It’s About People Not Technology
…II: It’s About Information Not Technology
…III: It’s About Business Not Technology
…IV: Nobody Uses the OS Anyhow
…V: It’s Platforms That Matter
…VI: And Anyhow, It’s Not Like an OS

And previously Jeremy Zawodny: There is no Web Operating System (or WebOS):

…Computers need operating systems but networks don’t (not at the OSI layers I’m interested in, at least). A Web Operating System is a myth propagated by people who either don’t understand the Web, don’t understand operating systems, or both.

…The web is a marketplace of services, just like the “real world” is. Everyone is free to choose from all the available services when building or doing whatever it is they do. The web just happens to be a far more efficient marketplace than the real world for many things. And it happens to run on computers that each need an operating system.

…The web is open and decentralized. Everything is one click away. Remember that.

Mashups are not toys. They’re a good illustration of this point… a hint of the future.

Read both.