Hi there - it depends on who you're targeting. Our client-side focus is on native developers looking for high-performance in web applications e.g. medical visualization http://vimeo.com/31970502 - the benefits of web applications are great, but performance is the current limiting factor.

We are likely to build support for NaCl in the future.

On the server-side, there is no requirement for a client to install a plug-in - it's for high-performance on the server i.e. semantic analysis, compute bound problems etc

Last point - we also think a lot about hybrid models where performance can be accessed on both client and server. This is much longer term, but the design of Fabric allows for it.