The point of Chrome OS, at least according to Google, is to cut out all the unnecessary bloat that traditional operating systems carry. If all you ever start your OS for is to fire up your browser, your browser might as well replace the OS. So Chrome OS does just that, without bootloaders, two rounds of hardware detection, etc. It's essentially the Chrome web browser running directly on a very slim Linux kernel.

Also, the way Google is implementing this, it's giving web apps direct access to hardware features that are normally unavailable to them - web games could use the GPU, for instance. That can help make web apps faster and more graphically appealing, and give them features that normally are only available to desktop applications.

Google has not introduced some amazing new service. It's trying to make existing products better.
Whether they'll succeed remains to be seen, but it's clear why Google is doing this. They're a web company, they make money online. If they can lure users away from the desktop and into the cloud, by offering them an equal or better experience, that means more revenue.

Seriously, what am I missing? Google wants your OS to be portable and availanle everywhere, but by creating bookmarks to Google mail/docs/wave/etc and saving your sign-in-cookie we have this already on more (st)able systems.