The playable characters in first-person shooters are almost always men. In the rare event that a female character is playable, she serves as an object of male fantasy and her interactions with the game world are still forced through the male-oriented lens described in the previous paragraph. Interestingly, playable female characters are usually presented in third-person action games (think Lara Croft) -- again reinforcing a visual power dynamic that in this case furthers the objectification of the female form by a predominantly male audience. Rather than the player assuming the identity of the heroine, she becomes a controllable other.
From the outset, Portal tears down FPS archetypes. The protagonist is a woman named Chell, but she's not the hypersexualized object of lust we've come to expect in games. Rather than skintight latex or a chainmail bikini, she wears a plain orange jump suit that is eerily reminiscent of those worn by prisoners in Guantanamo Bay. We're offered no backstory, no hint of personality. She is being held captive in a lab and is subject to teleportation experiments by the insane AI who operates the "Enrichment Center." As the player, you're never even aware that you're a woman until you catch a glimpse of yourself in the third person through a portal. The unobtrusive presentation of the female protagonist doesn't force a male gender perspective on the player as is the norm in FPS games.