Marissa Mayer is somewhat of an anomaly in the world of IT, haven risen through the ranks of many top vendors to their very helm. There's an ongoing debate about whether the C-level crowd is still a "boys club," but certainly it's unusual for a pregnant woman with lots of technical experience to be leading a multinational company.
Technical experience, I believe, is what really sets her apart. There are a healthy number of women with the ambition to rise to the top of the executive cadre, but few of them begin their careers as engineers or software developers. Without descending into a sociological analysis of why this is, the fact remainds that for whatever reason, these sorts of careers are more attractive to men.
In Tim's blog post, he expresses hope that Mayer can serve as a role model for many people who don't feel they'd fit into such a career -- not just women.