Jeffrey Zeldman, in his Nov 20, 2007 article Understanding Web Design on A List Apart, has an eloquent definition of web design:

“Web design is the creation of digital environments that facilitate and encourage human activity; reflect or adapt to individual voices and content; and change gracefully over time while always retaining their identity.”

I think this succinctly captures the big picture of what web site design is all about, and encapsulates Web 2.0 as well in the middle of the definition.