Email is pretty risky itself but using google is much worse.
They know who you are, what you like, who you associate with. And if you also use(d) other google services besides the email, they have a very complete picture of you. They probably know much more about you than your closest friends, relatives or spouse.
When you get a large amount of information about a person, it's relatively easy to probably know things like, sexual preferences, income, health record, your political leanings. Also, it makes one more likely victim of identity theft.
Most people think that because they're not spies or diplomats or even executives of large companies, they have little to hide online. Yet most of them don't live in glass houses and would object to you walking around their premises.
And it's the little things you do that might turn out to be dangerous or even things that you are. One scary example is your ancestry, during the second World War in the 1940s in Germany it was not a great idea to have Jewish relatives, you probably ended in a death camp. Similarly at the same time being in the United States and having Japanese relatives got you in an internment camp.
We've all seen on TV how detectives piece together puzzles of small unimportant seeming details. What happens online stays online, gets aggregated and sold, stolen, exposed. Different countries have different laws. Your next government might have radically different policies. What would you do if you knew the secrets of your competitor/enemy?
