I've been using GMail for a while, since Dale invited me in. But I noticed something different only today.
The links that you click on to read your mail, aren't HTML links at all. There is a slight difference in behavior—When you click on a normal HTML link, the action is triggered when the mouse button is released. In GMail, when you click on your mail, the action is triggered when the mouse button is pressed.
I'm surprised that it took me this long to discover this discrepancy. It also made me appreciate how different GMail is from ordinary web applications. I think I can argue that GMail is more like a Java applet then an HTML page.