Kitty's thematic principle mirrors the central theme of the X-MEN. Ostracized, she starts off alone and untouched; there's also the flipside of this theme, in which we can explore how much of that loneliness is self-inflicted. Audiences sympathize quickly with solitude and loneliness, especially in the middle of populated areas.
She's very much like Rogue, except her power is more easily depicted. Her powers are also a reasonable counter against any threat. We might find it hard to see a woman battle toe-to-toe with a man, and CGI flames never look realistic -- but we totally buy a girl who could phase her way out of most problems. Not to mention we could showcase her powers being used against her, such as preventing her from phasing back in by occupying her space with another person, maybe someone she cares about. The extreme of this is being permanently incorporeal, which opens up the possibility of ghost story tropes. Kitty's power it simultaneously soft and feminine and has the capacity to do serious damage if applied well, or as an extension of emotion (this is Storm's trope: weather mirroring her feelings).
A young girl appeals to both sexes, in that girls (and some boys) can identify with her, and most boys (and some girls) will want to see her do well/rescue her. Kitty is vulnerable, smart, and most of all plucky. Being younger she is beset by more problems, including a lack of respect -- which translates to more opportunities to show off her true mettle. <-- This is where most characters live and die:
being forced through a crucible.
With the other X-Women, they're not quite as vulnerable. Most of them have final boss powers, and they are established clearly and at the beginning. Kitty's powers can be teased out, thus her efficaciousness can build and build with surprising magnitude.
I like the idea of Ellen Page due to the Ratner notoreity, where the tough part of being a mutant is a symbol of the tough part of being in proximity of Ratner, as a woman, lesbian or not. A Page story could be crafted as a re-acquisition of power and dignity, that we will not be diminished by callous forces of a non-sympathetic society.