Why isn't it an OS in the true sense? It is a linux distribution at least. I would call it an OS.
The prospect of Chrome OS is that people would use this primarily on a secondary machine, since this makes extensive use of Google's service which all live in the cloud.
If you want more robust applications (or play games) or actually want to do real work, then this probably isn't going to be of any use as a primary machine since the Chrome OS is supposed to keep application installs at bay.
But it is also extremely early in development and many things can change between now and its official release, so I'm holding out my judgement until a more fully formed version is out and about.
I think what's special about Chrome OS is that it's simple. What do most people do on there computers, and I mean the average person. Check email, use Facebook, maybe play some flash games, read the news. That's about it. Chrome OS does all that stuff. The one fault I see so far is how to sync stuff like iPods, iPhones. And take pictures of cameras. That's the two things I don't know how Google will implement that might make or break Chrome OS. For people that couldn't live without the full desktop experience Chrome OS will be a perfect OS for there netbook. Also, think dual boot. Boot into Chrome OS and boot into your other OS when you need to get some work done. If you just want to use a web browser Chrome OS will be great.