The most sensitive part of your online Google existence is almost without question Gmail, and Google's working to make that more secure than ever. In response to revelations about the mass surveillance performed by the NSA, Google is reportedly looking at implementing more complex encryption tools, possibly including the very secure Pretty Good Privacy (PGP) encryption, in Gmail.
According to Venture Beat, Google has "research underway to improve the usability of PGP with Gmail." Their source notes that end-to-end encryption is best from a security standpoint and is currently compatible with Gmail, but implementing it on the user side requires substantial effort that most users aren't capable of, or at least willing to approach.
Gmail currently relies on SSL/TLS for data transmission through Gmail, though the encryption is nowhere near as strong as that of PGP. There's also the matter of advertising — if your email on Google servers is encrypted, that means that Google's efforts to run matching ads against it would be thwarted (as much as that might upset Microsoft's ad men).
It's worth noting that this is still in the exploratory phase, but given the outcry over the NSA's reported tapping of internet traffic for mass monitoring we wouldn't be surprised to see it implemented in short order.
Source: Venture Beat