My friend and I spent a couple weeks just trying to get the developer tools to work. We tried on Ubuntu, OS X, Windows, and Arch Linux. We weren't able to get started on a single platform. In the process we came across bugs that were several months old that haven't been fixed, even on their supported platforms.
Another thing to worry about is whether NaCl will take off. We can't know how committed Google is and how many users will install the plugin. It was supposed to be enabled by default in Chrome 6 (I believe) but it still isn't enabled in 7.
NaCl is still promising (especially Portable Native Client) but Google has to get it working first.
One of the problems that needs to be fixed is that LLVM bitcode is kind of bloated, something like 6x as compared with native code. Another issue is that bitcode is not platform independent. Things like pointer sizes and exception handling are dependent on the target you are compiling to. Thus, they have to define a bunch of specifications for PNaCL programs so that they'll run everywhere.
It will definitely be interesting to see if anyone uses it. Like you said, one killer app will do the trick. I'd like to see Google Earth reimplemented for PNaCL, or any Google project for that matter. That would give more confidence that this is a project that is going to be taken seriously within Google.