Gone are the days when every Google service was separate and unique and not interconnected. Nowadays, in our hyperconnected phone access based world single sign ons are only a tap away
Google has had their single sign on policy for a while now, and it keeps getting more ubiquitous each day. Their latest incarnation sports a home page which says "One Account. All of Google". It keeps your notifications, your YouTube videos, your e-mails, pretty much everything you do on the internet.
While Reddit might call itself the Front Page of the Internet, Google is the company that owns the paper. Google services provide us with the ability to do pretty much anything and most of it brilliantly (except you Google+). Youtube, Google Books, the Play Store, Maps. All those services can be accessed on one machine with one password. Its fair enough to say that Google has a ton of harmless and yet potentially horrible information about it.
The question is what's the real problem in having a single sign on policy on your devices. I will answer the question with an anecdote.
"A student, Aaron asks his friend, Penny whether he can use her computer to check his e-mail. Penny acquieses and allows him to check his account. Aaron signs into his account and checks his mail but forget to sign out. A few days later Aaron is checking his history and he sees a few sites he hasn't visited. He checks to see which Computer is loading up these results and finds out its his friends. The point of this story is at this point Aaron has unprecedented knowledge about Penny."
Our web history is assumed to be private, much like what we watch on TV or the people we meet. As Google becomes more ubiquitous the information we can obtain by a simple mistake is a cause of concern. What if in the future Aaron can check the places Penny has been or the chats she had with people.
We are forgetting at this point how much Penny knows about Aaron. She has access to what videos he watches, the things he searched for, his chats, messages etc. In the future she might know the temperature of his home (Google acquisition of Nest) or the time he reaches home from work (Google Now comes to Google Chrome).
One could validly say that it was Aaron's fault for not signing out of his Google account but that is beside the point. Humans are quite capable of mistakes. Unfortunately, as one company continues to grow we become more and more dependant on it.
Living in a world of single sign-on puts a lot more information at risk with the change of a single password. Your password is quite literally the key to everything in your life.