I was a little surprised that they haven't included anything about spam or gamification. One core advantage of pagerank is that it's (relatively) hard to get links from high-authority websites. I can't force whithouse.gov or cnn.com to link to me. If you rely on time-spent-on-page from millions of users and treat everyone equally, how to you stop spammers from faking millions of hours spent reading their content using spoofing or bots?