In the very early stages of Portal, the orange and blue portals are set up so that you can see yourself passing through them. Listening to the audio commentary, you learn that the developers created this room so that you, the player, would understand how the portals really worked; you needed to understand that orange and blue were two sides of the same coin, and that you were passing through the coin, no matter where its sides lay.
But I passed through the portal, saw myself, and understood something else entirely. "HOLY @&*%, I'M A GIRL," I shouted happily, to the eternal annoyance and confusion of my roommate. ("What, you didn't figure that out twenty-odd years ago?")
Overwhelmingly, fixed (as opposed to characters you can fully customize, as in an MMORPG or recent BioWare titles etc.) video game protagonists are straight, white, and male. This is not news. Sometimes there's a strong narrative reason for a character to be the way s/he is; other times, not so much. Sometimes the player character and his fixed narrative have a strong place in the game (any iteration of Metal Gear Solid, for example) other times, the character is more transparent and the player is truly conceiving of him- or herself in the first person.
And admittedly in Portal, the fact that you are Chelle doesn't matter much. Sometimes you catch a glimpse of yourself, but the game exists primarily in the first person and every player I've ever known felt that he or she was in a one-on-one battle of wits with GLaDOS. The character has no back-story given, so you write your own.
But: Portal presents a game where nothing about the player character really matters, so they flipped the coin and it came up female. And that's something we really don't see often, and definitely not enough.
I've been working on posts around this topic for a month, and keep coming up with thousands of words, rather than the few hundred which are suited to a blog post. Luckily, a blog gets to keep going. I'll write more about the whys, hows, and what-to-dos another time.