Back in June, when the contents of Edward Snowden's cache of NSA documents were just starting to be revealed and we learned about the NSA collecting phone metadata of every American, many people -- including President Obama -- discounted the seriousness of the NSA's actions by saying that it's just metadata.
Lots and lots of people effectively demolished that trivialization, but the arguments are generally subtle and hard to convey quickly and simply. I have a more compact argument: metadata equals surveillance.
Imagine you hired a detective to eavesdrop on someone. He might plant a bug in their office. He might tap their phone. He might open their mail. The result would be the details of that person's communications. That's the "data."
Now imagine you hired that same detective to surveil that person. The result would be details of what he did: where he went, who he talked to, what he looked at, what he purchased -- how he spent his day. That's all metadata.
When the government collects metadata on people, the government puts them under surveillance. When the government collects metadata on the entire country, they put everyone under surveillance. When Google does it, they do the same thing. Metadata equals surveillance; it's that simple.