From Chris Bower's latest report on the state of the progressive blogosphere:
It has been over one and a half years since a new blog has broken into the "short head" of the national progressive blogosphere, whereas not long ago new members of the "short head" used to be fairly common. Over time, their appearances slowed to a trickle, and now seem to have stopped entirely. Back in October of 2005, Glenn Greenwald's Unclaimed Territory might have been the last great individual breakthrough. Now, even Glenn Greenwald has gone institutional and blogs for Salon.com. The entry costs to the "A-list," the "top tier," and the "short head" have simply become too high for individuals to sustainably break through on their own. A caste system is solidifying and a new establishment is crystallizing.
Now, the flipside of this is that the existence of high-traffic multiply-authored blogs means that it's now possible for a new blogger to rise to prominence without that meaning a new blog becomes popular.