For anyone not freaked out about the NSA just having metadata on every email you’ve sent and received in Gmail, make sure you take a look at this little project/experiment some MIT students put together. The graphic you see above is a diagram of who I email, how often I’ve emailed with them and how and if they are related to other people I’ve emailed with. I removed the name labels, but you can view the diagram with names as well. This is approximately 10 years of email. So yeah, just having metadata can tell someone a lot. MIT lets you delete the info. The US government does not.
BTW, something I figured out was that because I had Gmail (on the web) configured to only give access to my 1000 most recent emails via IMAP, I had to remove that limitation in order to give this project access to all of my email.