“If you’re signed in, we may combine information you’ve provided from one service with information from other services,” Alma Whitten, Google’s director of privacy for product and engineering, wrote in a company blog post. “In short, we’ll treat you as a single user across all our products, which will mean a simpler, more intuitive Google experience.”
The privacy argument against this is now that everything you do on one Google product is provided to every other Google product, you’re just being watched and analyzed a lot more. The things I view on YouTube will affect my search results and advertising on Search, the things I look up on Google Maps will influence my Google+ account.
The changes take effect on March 1st, Google says it will send out emails to users explaining the changes and how totally awesome they are for everyone.