One of the question that comes up a lot in the blogs I read is what's causing the momentum swing towards RIAs. So often I focus on the examples (demos, companies, etc) and the benefits (good user interface, better designer/developer workflow, rich media, etc) but I haven't spent much time on what's actually causing people to care about rich Internet applications. That's possibly a more important question because looking at the why is a valuable metric for helping decide what's important in a space that is rapidly growing.
The bulk of the excitement comes because [with RIAs] we're basically talking about the next generation of web applications.RIAs really started getting adoption because of Web 2.0. Web 2.0 brought about the perfect storm of people and requirements to generate excitement in new kinds of applications. We started seeing the ease of delivering in a browser and as we started throwing more and more functionality into browser-based applications we ran into the limitations. At first it was just a limitation in the user experience. People wanted something richer than the text page-model of traditional HTML. They wanted the applications they were building to look like other applications on the desktop. That brought about Ajax, Flash,
But just as browser applications have evolved, so have RIAs. They encompass a lot more than those original richer interfaces in the browser. Now the architectural aspects of the original RIAs are finding their way into traditional software and taking the RIA name with them. Look at Windows Presentation Foundation. With XAML as the UI language and .NET for the logic it's adhering to traditional presentation/logic separation but with traits that are very RIA-centric such as a good designer/developer workflow, the incorporation of rich media, and a heavy focus on user experience. I also think WPF has made huge leaps in the web direction from previous versions of .NET. RIAs are ultimately about creating web applications regardless of they are in browser or out of browser web applications. The other obvious direction in the out of browser is Adobe AIR which took traditional web technologies like Flash and Ajax and provided developers functionality that didn't exist in the browser. Again, it's an out of browser application but with all of the traits and features that make for a rich Internet application. All that's changed is the delivery model.
Ultimately I think RIAs are a broad category that have begun to touch all parts of the software industry. But the bulk of the excitement comes because we're basically talking about the next generation of web applications. Some will continue to be delivered in the browser, other's will be out of the browser and others will run on devices or require some powerful hardware (Microsoft Surface). All of these applications keep most of the benefits of the original browser-based web applications in some way and are appealing to web developers because of added richness, more powerful programming models, and more features. Therefore I'd say the same things driving the web are driving RIAs. The two worlds are very tightly aligned and the number of solutions out there means developers can pick and choose RIA technologies to fit their needs. Keep the users in mind, that's all that matters in the end.