This doesn't even scratch the surface of what Google could show you. If they showed us the extent of what they can actually infer from our activity, some people would probably shoot their computer, burn their router and modem, and never look at a screen again.
It can probably build a very extensive list of skills (what languages you know, spoken or programming), preferences (habits, sexual, etc), hobbies, a full social network map (from emails and social networks), character (from analyzing emails, searches, etc), amongst other things.
I'm not necessarily saying that this data is already compiled somewhere - but Google being a data-oriented company, they could most likely do all this if they really wanted to.
Why would they let you see all their data unless there is a legal obligation to do so? It would just freak people out and they'll start looking for ways to anonymize or find and use another search engine/email/etc.
In other words given all their options and incentive they don't seem to get any benefits from really exposing all the data they have about you. One can argue that is really they most precious resource.
They have some incentive to expose some to make you feel like you are somewhat in control and they are somewhat open and transparent.
That is far less than what Google's recommender system actually knows. But it is open question to an outsider whether Google interprets what it knows, or of the knowledge is all a pike of correlation matrices that are opaque until the ads pop out.
One thing that Google knows: other accounts you have with non-Google services. For example, when you setup your G+ profile, they offer to link a Twitter account that they suspect is yours.
Twitter used to offer an api to lookup a user by email. So this does not have to be something they figured out by parsing your Inbox for Twitter signups.
