Excellent post on the Social Software Weblog, attempting a comprehensive definition of Web 2.0 Do not miss it!
Of particular interest to marketers and branding: the "inversion of control" in the relationship between business and community. New tools that are aggragating power away from monolithic structures/communications toward "hyper-customization" and individual control. According to the author, this unfolds at the very basic level at which culture and meaning is constructed:
we need not wait for “culture” to be handed down from on high by broadcast media. We create culture; we are culture.