While doing some research today, I saw a lot of posts that look at another key aspect of "Web 2.0": It's more than just user-created content as opposed to corporate-created content, and it goes beyond "conversation" and further into technology.
One of the key elements to this whole Web 2.0 thing is the shift of power from content-creator to content-AGGREGATOR. Not only does the end-user decide which of your messages he will read, he also now has the power to determine how good, relevant and important your content is. (And don't think the search engines aren't on to this too.)
The pressure's on to create content WORTH reading/viewing/ listening to.
(JaffeJuice has the best post touching on the topic -- and from the original source, "The Long Tail" - see Chris Anderson's blog.)