http://www.technologyreview.com/view/419965/the-search-engin...

The content he’s blocking includes everything from “Made-for-Adsense” sites that offer no content at all to sites that offer only, in Weinberg’s judgment, low-quality content designed specifically to rank highly in Google’s search index.

“Usually the stuff I’m blocking is pretty straightforward, i.e. not really on the line. The few that have been somewhat on the line are usually big sites, and I’ve asked users or they have been driven by user reports followed by my subsequent investigation,” says Weinberg.