The internal cool thing is that it's one of the first prominent uses I've seen of the XmlHttpRequest technique of going back to the web server for more data while the user interacts with a page... he latency of web UIs, in which everything you do is a slow round-trip that requires completely refetching and rebuilding the web page, is one of the reason web UIs feel so clunky compared to native GUIs. Google is very publicly raising the bar on the quality of interfaces that people will expect from web pages.
The first prominent use of XmlHttpRequest that I encountered (in my admittedly limited range of web browsing) was in the once exclusive but now culturally irrelevant Google social side project, Orkut. After this experiment, though, the technique really fell into place with GMail.
If you are a web developer, even if you don't need any more mail space, get a GMail account now. In one blow, Google's engineers massively raised the bar on what we should accept from a webapp. Here's a highly regarded, mainstream application that is responsive, with a dynamic UI and even comprehensive keyboard navigation, but that still feels like a web application.
We're entering an interesting age for web applications. Netscape 4 is finally dead for all intents and purposes, and support for advanced DHTML techniques across adopted browsers is finally widespread enough to justify using them in mainstream applications.
Looking at the web applications Google is producing, I can comfortably say (and I'm almost never comfortable making technological predictions), that if you develop web applications and you aren't looking today for ways to include dynamic interface techniques like those made practical by XmlHttpRequest, you're going to end up losing to someone who is.
(Note: Since I'm linking to Joel, who puts that little advert for Fog Creek at the bottom of his posts, I feel I should add that the company I work for, Atlassian Software, makes a terrific product called JIRA for software project management and bug tracking. It's web based, affordable, and runs on your own server, and it's now available for Windows, Unix, Mac OS X, AS/400, really pretty much anything that can run Java. Check it out now — there's a free online trial!)