This is a bit off topic, but it's my blog and my last week here so bear with me. I recently watched The Adjustment Bureau, a sort of sci-fi romance where the world is regularly nudged away from extinction by a covert bureaucracy of otherwordly chaperones who dress like 1950s pulp detectives. The female lead, played by Emily Blunt, is a variation on Nathan Rabin's "Magic Pixie Dream Girl," concept, defined as a woman "exists solely in the fevered imaginations of sensitive writer-directors to teach broodingly soulful young men to embrace life and its infinite mysteries and adventures."
I think Blunt's character actually diverges from the archetype in that (spoiler warning) male protagonist Matt Damon agrees to dump her because doing so allows her professional career to flourish. Later on, she willingly gives that up knowing what she's sacrificed -- but Damon's character does as well. Damon is the protagonist, and the Blunt character's personal struggles are of little interest to the film, but the actual plot complicates the archetype a bit.
As the Onion writers later note, the key offensive quality of the MPDG is, like the Magic Negro, subservience: She exists to lead the male protagonist to happiness/catharsis.
Like the Magical Negro, the Manic Pixie Dream Girl archetype is largely defined by secondary status and lack of an inner life. She's on hand to lift a gloomy male protagonist out of the doldrums, not to pursue her own happiness.
It's prominence as a cinematic archetype, I think, stems from the fact that it's the ultimate female fantasy of a particular kind of "nice guy" overrepresented among artsy men*. She's on hand to "lift a gloomy male protagonist out of the doldrums" because male writers are often the gloomy male protagonists of their own internal dramas.
Amanda Marcotte has done extensive blogging on this, but the basic idea behind the "nice guy" is that he's a dude who believes his lack of success with women is due to the fact that women don't "appreciate" his niceness. More honestly expressed, what this means is that the "nice guy" is a person who feels sorry for himself because his particular form of passive aggressive romantic coercion is ineffective. Despite his supposed "niceness," the nice guy actually retains a high level of internal contempt for women, who don't appreciate him because of their internal shallowness or attraction to "jerks." That this attitude is not in and of itself particularly nice is beyond the nice guy's comprehension.
My theory is that the MPDG is a fantasy molded from the clay of an infinite number of adolescent rejections from the women of their youth. Precisely because the relationship never reaches the stage of genuine intimacy, the MPDG remains a two-dimensional projection of the desires of a guy who is progressive enough in gender matters to want a woman who is "interesting," but not one that has an internal life of her own beyond the superficial qualities that made her "cool" and "not like other girls" to begin with.
Key to the MPDG is that the concept reflects the gender-based hostility of the nice guy. She frequently suffers from a form of (mental) illness, because this both proves that she needs the nice guy and shows why he has such a hard time acquiring her. Even if she's not sick in some way, she is defined by some kind of glaring emotional vulnerability that makes her, in an abstract sense, a damsel in distress who needs rescue. Under the circumstances, the nice guy's qualities become as heroic as he imagines them to be. She often suffers cinematically, because she refuses -- like the unattainable women of the nice guy's imagination -- to recognize just how good for her he is.
Just as with the Magic Negro, though, the insidiousness of the MPDG archetype lies in the way the creator assumes that their characters are progressive. These characters are in a superficial sense positive in that they're usually protagonists or allies of the protagonist, but the purpose of this is merely to assuage guilt and provide the unparalleled sense of comfort that comes with the knowledge that everything is in its proper place.
*Though I think this is probably less an indication of current nice guyness than the emotional remnants of it. Lots of guys go through a nice guy phase in adolescence, most grow out of it by adulthood.