Marc Canter had it absolutely right when he cautioned about the fuss over Ajax.
Perhaps Ajax is a meme more than a ‘thing’, and like all good meme’s something that is spreading because the environment is ready for it.
When I first used Gopher or WAIS and then downloaded Mosaic I was impressed by the architectural simplicity of Internet applications, so much so that I stopped being an architect and started working on web stuff.
Here was something in computing that was seemingly a retrograde step – one window instead of many. I spent most of my day at the time in front of a twin screen CAD application that had several hundred palettes. But because that one window opened onto a world of other computers, like a unix terminal, it was so much more elegant. (My favorite new experience with UI has been finally using VI, a text editor with Zen-like elegance that has evolved over 30 years while maintaining simplicity).
In fact the one window paradigm has persisted in an almost inevitable way, as the parasites of the web have driven people to use pop-up blockers, thus keeping it from becoming more like windowS, plural, apps.
The chunkiness of the web continued when forms were introduced, and people started to create applications, but with the new restrictions of statelessness and non event-driven UI.
Restrictions are sometimes held up as the mother of elegance and the web had all of them: bandwidth restrictions, browser compatibility restrictions, restrictions maintaining state, procedural user interface restrictions and the one window paradigm.
From this, the web has, in my opinion, improved the quality of software design.
But the web now exists in an environment where some of those restrictions are easing and perhaps the time is right for event driven UI (which as Marc pointed out, has been available for years with Flash, but just within the wrong community) so perhaps Ajax is, like better bandwidth, a natural evolution.
And, unlike the poorly thought out attempts at useability standardization, Ajax is an elegant way to lay down some specifics for richer web apps – even if there is absolutely nothing really new in it.