Read it for yourself and see what you think, but for what it's worth I think Miller is right about some points, a little off-base on some others. She's right, of course, about the abysmal quality of 99.9 percent of the material that finds its way into book publishers' slush piles. It's simply not true that there is a sizeable universe of unpublished masterpieces languishing in obscurity because of the blindness of the editorial gatekeepers.
She's also right, I think, when she says that, over time, new kinds of gatekeepers will inevitably develop to help readers sort through the flood of newly self-published manuscripts being enabled by the new online technologies. We don't yet know what form those new gatekeepers will take, but they will identify the blockbusters of the future and, by their neglect, consign the vast majority of self-published books to more-or-less complete oblivion.
But Miller is a bit off-base, I think, in taking a somewhat monolithic view of book publishing and readership. She writes, in effect, as if the entire industry consists of novels (and perhaps memoirs) that are suitable for a broad, general public and that are competing to attract that kind of readership. In reality, many, many books are nonfiction works aimed at very specific niche audiences--and this is where the new self-publishing technologies can play an important positive role.
There are thousands of topics that are of interest to small but devoted groups of readers who would be willing and able to buy books about those topics, generating sales not in the tens of thousands but in the hundreds. The new technologies make such "long tail" offerings more economically viable than in the past. And the Internet should make it possible for interested readers to find out about those niche books quickly and easily--without having to randomly wade through the flood of slush that Miller envisions as the brave new world of publishing.