Rich Internet Applications, in my experience of building large call center applications (yes! web-based), are a mix of page-centric HTML and the GUI applications. They trying to achieve the rich interactivity of GUI applications in a web scenario.
As we all know, 80% of usability is navigation. RIAs are breaking the paradigm of "navigation metaphor." Moreover, navigation is changing its meaning -- it has got many visible layers now.
The first layer has closed or partially open windows (sliding windows). These metaphoric windows contain information needed for successful completion of the immediate sub-task. These can take shape of a tooltip, a layer for quick selection, etc. It can also be quickly show the contents so that users can make a decision. This is information "on the fingertips."
The second layer has open windows. This layer consist of information or subtasks that are needed or performed by the user "regularly." For example, in a call center application, name of the customer is needed all the time by the user to address the user many times during the call, while the user also changes the customer address/phone numbers most of the times during the call. This navigational layer must always be visible and "accessible."
The third layer is a door that leads to another room - usually another page (or screen, or pop-up window, or similar artifacts) where the user must "go" to perform a task. These places are usually reached when the user performs tasks that are either not regularly performed or have information that is not needed in most scenarios. This layer is also usually reached navigating through layer two, however, shortcuts for advanced users do exist (depending on the personae).
In such navigation scenarios, the challenge is that the information and tasks must be findable on a screen (or page) vs. finding them on other pages. The scent of information through appropriate grouping, visual hierarchy, and spatial placement of artifacts on the screen gets very high importance.
My questions are: 1. How much support do popular webserver platforms such as Weblogic, Websphere, or Macromedia's JRun provide for RIAs? 2. How do RIAs cope with thousands of users logged in together, asking far more data than simple cookie based applications (page by page navigation approach)?