Google's idea of an operating system that consists of only a web browser sounds interesting, and unfortunately, very restrictive. There are a staggering number of applications out there that people use on Windows/Mac/Linux machines -- any of which would give enough reason to avoid switching operating systems.
The fact that you cannot run these applications on Chrome could be a huge contributing factor for the failure of the operating system.
It sounds like Google thought about this, and decided that it makes sense to allow people to run their legacy applications on Chrome OS. It's still unclear exactly how this will work architecturally, but it will be possible using a technique they have dubbed "chromoting" -- a mix of the words "Chrome" and "Remoting".
You will not be able to actually run applications, you will just use a remote desktop connection sort of thing, to connect to your Windows\Linux\Macintosh computer and use it remotely. The data will stay on the remote computer, not on the Chrome OS computer.
Semantics, but still. Native Client will not allow you to run a regular application, I believe. It will still be sandboxed, so there are a lot of limitations.
After reading this post by Gary Kačmarčík (a Google engineer), it sounds like you will be able to remote into your desktop computer to run the applications you need. Smart move.