Marc Bächinger gave a presentation how to develop HTML5 browser applications.
You should use REST on the server side because that makes the server and the components of your application easier to reuse.
The main drawback is that you have (often much) more complexity on the client. This can be controlled by strict application of the MVC pattern.
Helper libraries and frameworks
Since the whole application now runs in the client, security is even more important since attackers can do anything that you don’t expect.