Excellent summary by Richard MacManus, the founder of ReadWriteWeb, on some of the recent shifts that affect Web Publishing. He lists 5 reasons but I would add a 6th (which actually reconciles his 1 and 4): the shift of value from Creation to Curation.
As he points out, we're moving to more casual forms of publishing and at the same time are hungry for quality. As more and more people embrace curation as a form of expression, they're exactly doing that, using their own particular expertise to identify relevant quality content and publishing more easily than through writing a long blog post.
Via Giuseppe Mauriello, Guillaume Decugis