If I was building an OS today I'd be building what Google just announced.
Like most heavy technology users I've been moving heavily toward hosted web applications over the past few years. I don't use Evolution or mutt anymore, I use GMail. I don't organize my photos on my laptop and use my own hosted Gallery, I use Flickr. I've never been a big office application user, but when I'm forced to open a Powerpoint deck, edit an Excel file or print out a Word document, I do it using Google docs.
I've also spent the past four or five or so years working on blurring the line between what's on your desktop and what's online. At Flock I worked to synchronize your bookmarks to online services and between machines, to integrate personalized web search into your desktop workflow and to make publishing media from your devices as easy as publishing text from your keyboard. At Songbird we developed APIs to allow web apps to interact with your desktop media player and APIs to let your desktop media player access content from the web. At Rdio I worked on similar things, from a slightly different approach, I don't think I can talk about them yet.
I'm really excited that Google has the balls (and the skills) to go all out. To commit to offering enough APIs to web applications to allow them to provide the same functionality and user experience as desktop applications would. This isn't the first time that this has been attempted, but I think this time it just might work. Just a couple of years ago when the iPhone launched and Apple announced that the only way to write applications was to write web applications users and developers rebelled. The iPhone browser wasn't capable enough. Google have taken the right approach by committing to improving the web platform to support whatever APIs are needed before shipping the product.
I'll never be running Chrome OS. I rely on too many specialized applications, but I am looking forward to when Flickr can pull photos right off my camera and GMail's offline features are widely tested enough to actually work right. Much of the innovation in Chrome OS will benefit us all.