Here's what you need to know about the substantive changes in the new policy:
1. Up until March 1, 2012, the data Google collected on you when you used YouTube was carefully cabined away from your other Google products. So, in effect, Google could use data they collected on YouTube to improve and customize the users' YouTube experience, but couldn't use the data to customize and improve user experience on, say, Google+.
2. The same siloing took place for your search history. Previously, Google search data was kept separate from other products. Even when users were logged in, Google promised not to share the information they gathered about you from your Google search history when customizing their other products. Considering how uniquely sensitive user search history can be (indicating vital facts about your location, interests, age, sexual orientation, religion, health concerns, and much more), this was an important privacy protection.
I used to have Firefox plugin that turned off my Google cookie unless I was visiting a service where I wanted to be logged in — that is, I could automatically log in to Gmail and Google Docs, but I wasn't logged in for searches, YouTube, and BlogSpot. It disappeared a few versions back. Does anyone know of a contemporary equivalent? Post it in the comments.