The most striking thing about the early Gmail patents is how exhaustive they were in attempting to anticipate every conceivable attribute of an email message that might one day be exploited for ad targeting purposes. In many cases it would be years before Google was actually able to make these ideas operational in Gmail. The first version of ad serving in Gmail exploited only concepts directly extracted from message texts and did little or no user profiling — this method would only be put into practice much later. Some attributes have still not been implemented today and perhaps never will be. For example, as far as I know, Google does not reach into your PC’s file system to examine other files residing in the same directory as the file you attach to a Gmail message, even though the patents explicitly describe this possibility.
Source: Jeff Gould on The Natural History of Gmail Data Mining