According to the ruling, "A markedly higher proportion of infringing-video watching may bear on plaintiffs’ vicarious liability claim, and defendants’ substantial non-infringing use defense."
Viacom lawyers are claiming that YouTube intentionally turned a blind eye to copyright infringement because of the traffic it generated, and they're hoping to show that YouTube officials knew, or should have known, the correllation between popularity and piracy.
This is essentially an end-run around Google's claim that they've done everything required by law by responding to DMCA takedown notices in a timely manner.
Viacom hopes to take advantage of the standard established in MGM v. Grokster that a business which is built primarily on piracy isn't eligible to claim a defense of 'substantial non-infringing use.' That would mean the DMCA defense doesn't apply.
Substantial non-infringing use was established as a defense for providing technology which can be used for copyright infringement in the famous Sony Corp. of America v. Universal Studios, Inc. case, also known as the Betamax case.
Google argued that providing usernames and IP addresses to Viacom raises potential privacy issues, but ironically the judge used a statement previously published by a Google software engineer against them.
In February of this year Google's Alma Whitten wrote on the company's Public Policy Blog "data protection laws should apply to any data that could identify you. The reality is though that in most cases, an IP address without additional information cannot."
Google will also be required to turn over copies of all the video that's been removed by YouTube for infringement. Google had requested that Viacom be required to request the specific videos relevant to their discovery. The judge decided this would be prohibitively difficult for the plaintiffs, and saw no harm in simply giving them access to all of it as long as they provide the required hard drives to store it on.
He did turn down a number of other requests that would have given Viacom access to a large amount of proprietary Google code. Judge Stanton felt that the value to Viacom's case would be minimal, while the potential harm to Google from disclosing how things like their highly profitable ad business work under the hood would be significant.