To me, Petra has the perfect all-around game and nice variety. She hits hard but not overly so. She moves well and is extremely fit for a girl of her age. She does focus well but can become a little vulnerable in close matches, like the final of the Nike event where she had a substantial lead, but ended up losing. That's about everything, I think. She's also very pretty and will most likely become the next "IT" girl if she makes it big.
I hope that helps.