I think the idea behind a 'web OS' is horrible. Web apps are good for certain things: mainly presenting and manipulating text. Once you get outside of that small domain, you need much richer interfaces, and the web just doesn't present that. In fact I'm not sure it should.
The browser is a great platform for certain things, but everything doesn't belong inside of it. It shouldn't be an OS. Web OSs aren't that attractive to the consumer - they can just as easily visit multiple websites in several windows, and they aren't that attractive to developers - web development is relatively easy, and you lose a lot of power once you start developing on some web OS API vs doing it yourself.
On one hand I have to agree that there was more than one occasion that I would get all hot and bothered with the thought of building a WebOS. Infact I flirted with the idea of building a Mobile Phone OS. Thinking that users will become more affluent in the future that they themselves will choose the mobile OS that suits them. And like most technologists I found myself out of touch with the world and reality in general. So the idea of a WebOS, as far as I'm concerned is a pipe dream. Too many things to consider such as the limitations of existing web standards and programming languages, browser limitations and the such. To hardware dependencies that require a native platform to run off of. Although that doesn't mean I won't stop dreaming :P
I agree that HTML+javascript has serious limitations. But eventually GUI complexity can be managed through good APIs (XUL, flex), and high-bandwidth apps can be delivered through plugins that use lower-level protocols (flash). The current state of these technologies still seem less-than-optimal, but I can see a lot of effort being put into making things more transparent and efficient.
I don't think it's a good fit for the web. The level of abstraction is at the wrong place. The web is an OS. The home page sites like Pageflakes are much more a WebOS than the sites cloning old-style desktops in Ajax.
I think there is potential in Web OS. But they can't be a defacto replacement of the desktop interface. I think the wufoo approach is better than say the youos approach.
The browser is a great platform for certain things, but everything doesn't belong inside of it. It shouldn't be an OS. Web OSs aren't that attractive to the consumer - they can just as easily visit multiple websites in several windows, and they aren't that attractive to developers - web development is relatively easy, and you lose a lot of power once you start developing on some web OS API vs doing it yourself.
I just can't see these services catching on.