What makes this different from just a hosts file or adblock is that it takes the most blocked sites and sends them back to Google for analysis.
The first thing that came to my mind was the numerous crappy one click hoster "link" sites that seem to consistently manage to make it into the top search results as long as you search rapidshare+anything
Most people however seem to be glad that this might finally mean the end of sites such as experts-exchange.com. This seems to appear in a surprisingly frequent number of programming related searches, only to infuriate the answer seeker with a subscription fee for the answer.
What a lot of people still don't know though, is if you scroll right down to the bottom on these pages, the answer was there all a long for free :)
Scroll down to the bottom on experts-exchange for the answer.