The ACLU took a close look at what this means for users on their technology blog here. Their main concern is that the cross-pollination of data allows what you do on one Google-owned site to affect what is displayed on another Google-owned site. Many people Google-search things every day that are highly personal and this information could be used to spawn targeted ads when using other websites such as Youtube. Further, advertisements could be spawned on other Google websites based on private e-mail content as well as search terms.
A final concern is with anonymity. The new policy will make it difficult for users to disassociate their accounts with certain sites from their real identities. Many people use these services with placeholder accounts not directly tied to their own names. The new policy will replace past names associated with your accounts with your real name so that users are represented by a single name across all of Google’s services.