Basically it boils down to this: If you are logged in to Google it might choose to combine information you have already provided in one of their services with the capabilities of another service. This isn't just agreeing to a single sign-in across all of the services offered by Google; it's allowing Google to use information that you have provided to any of their services to "improve" your user experience.
Although some hypothetical "average" user might already be using a single online identity for everything, it is far more common for people to take pains to separate, at the very least, their personal and business lives. And don't think that you can choose to opt out of this new policy; that's not an option. When it goes into effect on March 1, you won't be given a choice.
If you're thinking, "Well, all I have is a personal Gmail account and I don't really see a problem here," I hope you're not an Android smartphone user, because you had to provide the login information when you set up your phone. You know the one that you carry with you all the time, that is always logged in to Gmail, and that knows where you are because of that built-in GPS. Big brother is not only tracking you, but it's watching your online footprint, correlating things you do or talk about in email, analyzing your Web searches, keeping track of navigation that you have done, and cross checking the various components of your life that you have given Google access to, all in the name of "improving your user experience."
If you're a Gmail user you might already have noticed that you get ads at the top of the page that are targeted to you based on your searches and email content. The more information Google is able to obtain about you, the more carefully it can target its ads, increasing the click-through rate and charging more for its advertising based on the success of those targeted ads. That's one of the ways that Google funds all of the "free" services that are so popular with users.
But what we are talking about now goes well beyond that simple targeted ad. Let's consider the simple event of going away for the weekend. You mark in your Google calendar that you're planning on attending an event in a different city from home. Google starts sending you hotel, restaurant, and activity suggestions for that city. Perhaps it also reminds you that you have a half-dozen friends and family members in that city that you could visit.
It seems rather innocuous, but this also means that your Google+ friends in that city might also be told that you are in town, because you didn't tell Google to keep that piece of information private. In current Google fashion you will likely have to navigate a maze of screens and menus to determine what information is made available and what isn't, should you choose to turn features on or off. Perhaps those friends are also relatives who are now annoyed that you didn't call to say you were visiting. No one really wants that guilt call from their mother because she found out via Google that you were in town and didn't visit.
Okay, maybe that doesn't sound so bad. But what happens when it isn't your mother but your employer who, thanks to the fact that the Android phone in your pocket that you used to find your way to a job interview, is one of your Google+ associates who now knows that you just navigated to the location of your company's biggest competitor?
Not so innocuous anymore, is it?