To understand the huge importance of Buzz, we have to look beyond the surface. If Facebook adopts the same mix of protocols as Buzz (and it looks as if they will), we'll be officially in the post-platfom era.
By this I mean, there will no longer be singular, imposing applications that capture people. Instead, the web will be a meta-platform of content flows, unencumbered by actual applications. (Example: LiveJournal/Buzz integration.)
If I am right, this will be the end of silos, and of the platform as we know it.
(One thing I don't understand yet is why Facebook would actually do that, except maybe to battle Twitter.)