Face it, the scope of web applications is expanding. Text-based information sites will still continue to use regular XHTML, but a lot of new application types are appearing now that Rich Internet Applications (RIA) are a reality.
Personally, I'm standing behind the open-source OpenLaszlo platform. The main reason is that consumers are adopting computers and the internet for most of their multi-media needs. Nobody finds it weird anymore that you listen to music or watch a movie on your computer. Portable music players like the iPod go hand-in-hand with desktop applications to manage them, applications for home-recorded movie editing are included in most operating systems and Apple's Garageband brings awesome audio creation power to everyone. OpenLaszlo allows you to write RIA that incorporate sound, music, images, video and text easily, just look at what Pandora has done.
The main problem with RIA is their runtime platform. Flash is the most appropriate technology for it nowadays due to its market penetration and singular target. Sadly, there are major speed and stability problems. Anything beyond simple applications need careful architecting and exhaustive testing to run acceptably. Alternatively, Ajax may seem nice for certain effects, but it will always remain an uphill battle to track down all the browser differences and work around all the specific platform bugs. I thought we learned that lesson with DHTML six years ago. Also, Ajax also totally lacks any kind of multi-media capabilities.
I'm actually disappointed that the proliferation and integration of Java applets has failed. I'd trade any Flash solution for Java2D and Swing. If only the download size of the JRE would be smaller and the installation as transparent as Flash. Sadly I think that Java's reputation in this area has totally been compromised and that any user runs away as soon as the word applet is dropped. While Java Webstart brings a good alternative for full-blown applications for a targeted audience, it doesn't integrate transparently with the browser. Maybe Sun just has to repackage applets and give it another name, touting it as the next big thing and doing it right this time.
The recent canvas feature of Safari and Firefox looks promising. They seem mostly compatible and there's a standard in the works that specifies how they should behave. Maybe it could be used to build consistent RIA user-interfaces which use standard Ecmascript for the logic and only use Flash for the multi-media fragments. Now Internet Exlorer only needs to play along ... 
I wonder what's next for cross-browser and platform agnostic Rich Internet Applications. What are your predictions?
Update:
I added two movies, one with Wingz (which uses applets) and one with OpenLaszlo (which uses Flash). Make up your own mind which one is the most user-friendly. Note that I use this on MacOSX where Java is installed and integrated with the browser by default.
The movies require Quicktime and TSCC Codec, which you can download for free.
Wingz version (3.2MB)
OpenLaszlo version (2.2MB)