I am really excited about XmlHttpRequest being available in the major browsers.
One of the main reasons is that I think it really enables componentization.
Take the example of a portal, with 10 components on the screen. In the past the web application would have to round-trip to the server, and rerender everything. What a waste of time!
Now, with XHR, each component can talk back to the server if it needs too, and the rest of the page stays put.
Ok, more efficient, but is it that big a deal?
I think it is. The real power comes in when you think about having plugged in those 10 components from DIFFERENT SOURCES. That works just fine now, whereas it would be a real PITA as the portal software would have to coordinate everything in the past. Now, each component truly is its own component, and our clients talk directly to its server-side representation.
I expect to see a lot more ideas such as Y!Q, that make use of this.