How will her inevitable future sex appeal affect Arya?
I won't spoil anything down here, but this question is more directed towards book readers who know Arya's future.
The reason we love the character in the book is because she is a cute, defiant ten-year-old; the perfect little sister (or daughter) you would never need to worry about when she's out, who refuses to be the mouse in a dangerous world. As the novels progress, she falls deeper into darkness, but she is still not older than 11-years-old in the last published novel.
Maisie, on the other hand, turns 16 this year. This is the age that turns a girl into a woman (biologically, at least). She does have a screen presence that says "12-years-old", but how long can she and the directors hold onto to that? How long can she stay a defiant child rather than a defiant young woman?
Maybe my worry is for naught. I guess time will tell, but the fact remains, that if they choose to do all seven books (five are published), it will take 7- 10 seasons filming them, which ends up making her (and the rest of the kid actors) into pure adults. I don't worry much about the other characters, though, since I don't believe that their characters need to be seen as a child out of necessity. Arya has that need, however, as it puts weight on how small Arya, a child, is, but also how dangerous such a child can become, when mind is put to task.