Gmail is not really a Web 2.0 application. It's just email, but done right. There is no attempt towards buzzword compliance in gmail. Don't confuse synchronous XML requests with Web 2.0
Arguably the point of all Web 2.0 apps is just X, but done right.
The label's association with buzzwords, pastel colors, and flashy JavaScript is a side consequence.
But the same could be said of gmail - a definite web2.0 app.