Grant Skinner's 3 C's of UI design

Context

Ensure that the options and content presented to the user are relevant to their context. Showing a user too many options at a time will confuse him, hiding too many will obstruct his ability to get anything done.

Consistency

Maintaining consistent interface standards throughout your application will reduce your users' learning curve for your application. They can learn once and apply them throughout the app. This applies to virtually everything you create in your interface development process: interface widgets, keyboard shortcuts, metaphors, icons, placement of buttons, error message text, help systems, etc.

Communication
The sole task of any interface is communication. It must communicate to the
user the available methods of interacting with the data, in a responsive and intuitive manner. It must then facilitate the user's requests in as transparent a manner as possible

With an RIA the interface's environment will include a server and a browser. Most online applications also have an important branding or marketing component and it will be up to the interface team how to incorporate these elements without sacrificing the usability of the interface.